Ovarian granulosa-stromal cell tumors are characterized by trisomy 12. Eleven ovarian granulosa-stromal cell tumors including 1 thecoma, 2 fibromas, 6 fibrothecomas, and 2 granulosa cell tumors, were karyotyped after direct harvest and/or short-term tissue culture. Bilateral fibrothecomas from one patient appeared to lack cytogenetic aberrations: the remaining nine tumors were characterized by trisomy for chromosome 12. Cytogenetic aberrations in the two granulosa cell tumors were much less complex than those described previously in undifferentiated carcinomas; accordingly cytogenetic analyses might be useful in distinguishing these categories. The consistent occurrence of trisomy 12 in different varieties of granulosa-stromal cell tumors suggests a common mechanism of oncogenesis within this diverse group of neoplasms. That mechanism probably involves promotion of low-grade, orderly cell proliferation. 