There is $2.3 billion sitting in the investment portion of the nation's more than 15 million health savings accounts, so the question on the minds of many corporate benefits managers is where is all this money going?

For the most part, employees have invested it in mutual funds, primarily stock funds holding shares of companies with a large market capitalization or midsized firms with a middle market capitalization with “some convergence to index funds and lifestyle funds,” said Eric Remjeske, co-founder and CEO of Devenir Group L.L.C. in Minneapolis.

Average HSA account balances at the end of last year were $2,375, up 3.2% from 2012, according to the Year-End 2013 Devenir HSA Research Report. For the most part, HSAs remain small accounts. Usually, balances between $2,000 to $3,000 are required before the money can be moved from FDIC-insured cash accounts to investment accounts.

Therefore, there's no need now for more than between 15 and 40 mutual fund options to satisfy most account holders with higher than average balances, said Maureen Fay, senior vice president in the health and benefits consulting practice of Aon Hewitt.

With the strong performance of the investment markets, HSA investment account balances are projected to reach $3.8 billion or 13% of all HSA assets by 2015, according to Devenir. In 2006, investment assets comprised only 7% of all HSA assets.

Total assets — cash and investments — held in HSAs are expected to reach $29.7 billion by next year, according to Devenir.

With the investment options attached to HSAs, experts sometimes refer to them as medical IRAs or the health care 401(k).

People view them as a better option than the health care flexible spending account — pretax money typically deducted from paychecks for annual medical expenses — because “there's a lot more you can do with it,” said Todd Berkley, president of HSA Consulting Services in Minnetonka, Minn.

For example, a $10,000 HSA investment account balance in a mutual fund tracking the Standard & Poor's 500 index would have grown last year by about $3,300.

That $3,300 in investment income could have been swept into the HSA's FDIC-insured cash account and then withdrawn to pay for medical expenses.

Employees working for employers offering HSAs through large mutual fund families have the most choices.

Fidelity Investments' HSA clients have access to Fidelity's menu of 5,000 mutual funds, exchange traded funds, stocks, bonds “and all sorts of (decision-support) tools,” said William Applegate, vice president of Fidelity in Boston.

For customers who invest through Fidelity HSAs, the average cash balance is $2,200, and the average balance in the investment portion of an account is $11,800, Mr. Applegate said.

Meanwhile, astute investors, those with the highest account balances “are pretty engaged,” Mr. Remjeske said, often paying medical expenses out of pocket and letting the investments grow tax free.

“In five years, people can take all that money back out (of an HSA) and repay themselves for medical (expenses),” he said.

Some people have as much as $100,000 in an HSA account, Mr. Berkley said. They got there by rolling over former Medical Savings Accounts opened during the 1990s, maxing out employer HSA contributions, and their own annual contributions, paying medical expenses out of pocket, keeping receipts for medical services and letting their accounts grow.

“If you have that flexibility, then the HSA with an investment account makes a lot of sense,” Mr. Remjeske said. The few employees fortunate enough to have amassed a six-figure HSA balance may find they need more investment options and brokerage capabilities. “For employees with a lot of money, more choice is best,” Ms. Fay said.

Among the four employer-sponsored health savings accounts sanctioned by the IRS — HSAs, MSAs, FSAs and Health Reimbursement Arrangements — only the HSA offers the ability to invest.

The annual growth in HSA account balances shows the model has gained traction in the employer benefits marketplace since its inception in 2004. (Congress passed a law in 2003 to create HSAs, effective Jan. 1, 2004.) And the past few years have seen rapid adoption rates for HSAs among large employers in particular.

Chicago-based aircraft manufacturer Boeing Co. began offering HSA enrollment with the high-deductible health plan it offered to most nonunion employees last year, company spokesman Joseph J. Tedino said.

About 25,000 eligible Boeing employees enrolled in high-deductible health plans for 2014, an increase of 14% over last year. Data for Boeing's HSA enrollment were unavailable.

Boeing's HSA trustee, Utah-based HealthEquity Inc., imposes a $2,000 threshold before health savings account assets can be invested in mutual funds with no setup or trading fees. But there's no minimum to open an FDIC-insured HSA cash account.

As the company-sanctioned HSA provider, Boeing employees who sign up for a health savings account must use Health-Equity, Mr. Tedino said.

HealthEquity charges a monthly maintenance fee of $3.95 on all accounts, and offers as many as 38 mutual funds. Asset classes include bond, equity, sector and index funds. HSA administrators and custodians will customize the slate of investments to match a large employer's 401(k) investment offerings.

Employers, however, want to stay away from any fiduciary responsibility toward employees to avoid violating the Employee Retirement Income Security Act. “They (employers) don't want to have any input into how employees choose (HSA) investments,” Mr. Remjeske said.

As investment balances in HSAs grow, HSA administrators are starting to offer investment advice either online or through live counselors, paid for through underlying administrative charges or via a fee to the employee, Ms. Fay of Aon Hewitt said.

The administrators are “trying to figure out what level of investor they are getting,” she said.

With annual HSA contributions capped by the IRS in 2014 at $6,550 per family and at $3,300 for an individual, there's little sense in having more than one HSA account. Separate accounts would incur their own maintenance fees.

Some employers allow employees to work with an HSA administrator on their own, but that will likely entail higher fees since the employer doesn’t own the relationship with the HSA administrator, Ms. Fay said.

Making matters worse, “You don’t get the employer contribution, if you don’t choose the HSA administrator of the employer,” she said.

Looking ahead, education campaigns will play a critical role in informing employees of the eligibility requirements and how HSAs work, much like they did a generation ago when many employers switched from a defined benefit pension plan to a defined contribution retirement model with 401(k)s.

Marathon Oil, which last year offered its employees and retirees under age 65 a high-deductible health care plan option, launched a five-month-long communication campaign that included workshops, videos, staff meetings and print brochures, said Kim Moore, the company’s benefits manager for health and welfare.

The initiative included an employee benefits expo at the company’s Houston headquarters and an online interactive tool offered by Marathon’s HSA provider Fidelity Benefits Consulting, Ms. Moore said.

Three-quarters of employees eligible for the HSA attended workshops, two-thirds attended the benefits expo and 40% used the interactive tool to compare plan benefits.

In the end, 51% of Marathon’s eligible employees opted for the high-deductible health plan with the HSA option.

“We knew we had to actively educate our workforce on the advantages of an HSA, so we worked closely with Fidelity to create an effective education campaign,” Ms. Moore said.