Germline mutations in the BRCA1 tumor suppressor gene are associated with increased risk for the development of ovarian cancer. All such cancers thus far reported have been of the epithelial histologic type. We identified an ovarian dysgerminoma in a 16-year-old woman (proband) with a family history of ovarian cancer during a review of histopathologic characteristics of ovarian cancers from women enrolled in the Gilda Radner Familial Ovarian Cancer Registry. Mutation analysis of DNA from this patient's peripheral blood leukocytes revealed a germline BRCA1 mutation (3312insG). The mutation was also present in the mother with breast cancer, a maternal aunt and a distant cousin with ovarian cancer, and a maternal grandfather and an uncle with skin cancer. The development of the proband's dysgerminoma may be unrelated to her germline BRCA1 mutation. Alternatively, such dysgerminomas may be caused by BRCA1 mutations, but occur so infrequently compared with epithelial cancers that they are seldom identified. Analysis of a larger series of ovarian germ cell tumors may resolve this question.