SPRINGFIELD, Mass.—A CIGNA Healthcare Inc. manager filed a gender bias lawsuit Thursday that seeks class action status against the health care provider, alleging the CIGNA Corp. unit gives preferential treatment to men and discriminates against its female employees.

According to Bretta Karp vs. CIGNA Healthcare Inc., which was filed in federal court in Springfield, Mass., the Boston-based provider contracting manager, who has worked at the unit since 1997, said she was told she was denied a promotion last year because she “came across as too aggressive” in interviews.

Instead, the job was given to a less-experienced male employee, Bill O'Donnell, who subsequently informed Ms. Karp that her largest market, Vermont, was being given to a younger, less-qualified male employee.

When Ms. Karp complained, Mr. O'Donnell made “veiled threats” to Ms Karp, “such as reminding her that he would be writing her year-end performance review,” according to the lawsuit.

The suit seeks class action status, alleging that Ms. Karp's experience is part of a pattern of gender discrimination at CIGNA Healthcare.

“CIGNA's predominantly male managers hold female employees, including both (Ms. Karp) and class members, to stricter standards than male employees, and thus, female employees often receive lower performance appraisals than males for performing at the same level. Additionally, male employees more often receive favorable work assignments and other forms of preferential treatment, including the allocation of company resources,” according to the complaint.

The lawsuit also alleges a hostile work environment.

“Male supervisors and employees have harassed and intimidated female employees, have made it clear in various ways that they favor male employees and otherwise have created a working environment hostile to women,” according to the suit.

Among other things, the lawsuit seeks at least $100 million in compensatory, nominal and punitive damages and a restructuring of CIGNA Healthcare’s workforce “so that females are promoted into higher and better-paying classifications, which they would have held in the absence of CIGNA’s past gender discrimination.”

The law firm that filed the action, Sanford Wittels & Heisler L.L.P., represented female employees of Swiss pharmaceutical firm Novartis A.G. in a class action that settled for $175 million last year.

In January, the law firm filed suit in federal court in New York on behalf of a senior human resources manager at Toshiba Corp., seeking $100 million from a U.S. unit of the Japanese firm for alleged gender bias against women in pay and promotions.

In a statement, Philadelphia-based CIGNA said, “We have just received the complaint and are reviewing it. We are committed to diversity and equal opportunity; our workplace policies expressly prohibit discrimination in any form and we intend to fully defend against the complaint.”