Flow cytometric DNA ploidy and S-phase fraction (SPF) analysis was performed on 18 granulosa cell tumors of the ovary. Clinical and pathologic data from patients followed for an average of 10 years were compared to flow cytometry ploidy to determine its prognostic usefulness. Eleven (62%) tumors were diploid and seven (38%) were aneuploid. Aneuploidy was significantly associated with high stage, vascular or capsular invasion, and nuclear atypia but not with SPF or necrosis. Of the patients on whom follow-up was available, only 1 of 10 (10%) with euploid tumors died of disease, while 4 of 5 (80%) with aneuploid tumors died of their disease; this, however, did not reach a significant statistical correlation. SPF was not correlated with any clinical or histologic parameter. Aneuploidy in granulosa cell tumors is associated with other adverse histopathologic parameters and an apparent trend toward aggressive behavior.