We determined the proliferative index (PI) of 92 previously untreated advanced epithelial ovarian cancers using PCNA/cyclin immunostaining and image analysis quantitation. In this retrospective study, there was a relationship between tumor PI and 5-year survival. For patients with a tumor PI greater than the median, the estimated 5-year survival was 44%; for patients with a tumor PI below the median, the estimated 5-year survival was 15% (P = 0.003). This may partly reflect sensitivity to chemotherapy, as those patients with more rapidly proliferating tumors were more likely to achieve a pathologic complete response to platinum-based therapy.