The nuclear DNA content of 50 ovarian tumors initially diagnosed as granulosa cell tumors was measured by flow cytometry using paraffin-embedded archival material. The follow-up period of the patients ranged from 4 months to 19 years. Thirty-eight tumors were diploid or near-diploid, while 5 were aneuploid. DNA profiles of 7 tumors could not be evaluated. All 50 tumors were immunohistochemically tested for expression of intermediate filament proteins vimentin and cytokeratin and epithelial membrane antigen. The cells of all but 3 tumors expressed vimentin. These 3 vimentin-negative tumors were positive for cytokeratin and epithelial membrane antigen. They were highly aneuploid and though originally diagnosed as granulosa cell tumors, most likely represent undifferentiated carcinomas. Hence, only 2 typical granulosa cell tumors were aneuploid. In addition, frozen tissue samples from 9 of 10 granulosa cell tumors showed a DNA diploid content. Our results indicate that granulosa cell tumors tend to be diploid or have only minor ploidy abnormalities which is in line with their relatively benign character. An undifferentiated carcinoma should be considered in the differential diagnosis of tumors with a high DNA index.