In vitro activity was determined by primary culture of tumor samples obtained at surgery from 63 patients with Stage III ovarian cancer. These patients had completed at least 24 months of follow-up. Proliferative activity was measured after 3-hour culture by 3H-thymidine incorporation and metabolic activity by 3H-uridine incorporation. A large range of individual tumor activity was found; no correlation was present between proliferative and metabolic activity in the same tumor, the distribution of tumors with high and low activity was similar between histologic types, and the activity was not higher in undifferentiated tumors. There was a strong association between in vitro activity of the tumor and patient outcome (both clinical status and survival). On the basis of in vitro activity, a subset of patients was identified within subgroups with known amount of residual tumor; proliferative activity was a better predictor of good outcome in patients with no residual disease, whereas metabolic activity was better in those with more extensive disease. In these patients, a tumor showing high proliferative and/or metabolic activity (greater than 5000 cpm) was associated with poor survival.