After two decades, devoted patrons will have to forget about the Forgotten Woman. The national chain, known for its larger-sized women's clothing and its high-end evening wear, has filed for bankruptcy and is closing. In its heyday, Forgotten Woman had some 30 stores across the United States. The original store, which opened about 20 years ago at 888 Lexington Avenue (66th Street) will close by the end of the month. The Rockefeller Center location at 60 West 49th Street has already closed.

Rick Marech, executive vice president of Fox, the liquidating agent, said that the business had been highly successful in New York, but that it was ''ill equipped'' to handle the added business after expansions into markets like Houston, Chicago and Beverly Hills, Calif.