It has been shown that the monoclonal antibody Ki-67 reacts with a nuclear antigen that is expressed only by proliferating cells. The feasibility of using image analysis to quantitate Ki-67 staining (proliferation index [PI]) of epithelial ovarian cancers was investigated. The PI was determined in 50 advanced-stage primary ovarian cancers. Frozen sections were immunostained with the Ki-67 monoclonal antibody, and the PI was calculated using static image analysis. Among 35 stage III ovarian carcinomas, the median PI was 8.9%, compared with 17.7% in 15 stage IV cancers (P = 0.06). There was no relationship between PI and histologic grade. The median survival time of 32 patients whose cancers had a high Ki-67 expression (> or = 7.5%) was 16.8 months, which differed significantly (P < 0.01) from the median survival of 31.5 months observed in patients whose tumors demonstrated low Ki-67 expression (< 7.5%). Quantitative image analysis of Ki-67-stained fresh-frozen ovarian cancers may provide useful prognostic information. Further studies are warranted to investigate the relationship between Ki-67 expression and other known clinicopathologic and genetic features of ovarian cancer.