The proliferative activities (3H-thymidine labeling index, LI) of 72 primary ovarian cancers and 76 metastatic lesions from untreated patients were evaluated. Overall, median LI values for primary and metastatic lesions were similar (7.8 vs 7.0%), but cell kinetics significantly differed in metastases from different sites. The LI of the primary tumor was unrelated to pathologic stage and histology, but was significantly correlated with histologic grading (P = .014). The prognostic relevance of LI was assessed for 43 untreated patients at stage III-IV (90% with bulky residual disease), treated after staging laparatomy with five cycles of cisplatin or of carboplatin. For 19 patients the LI was determined for both primary tumor and metastases, for 15 for the primary, and for 12 for the metastatic lesions. Complete remission (CR) was unrelated to pretreatment LI, although a trend toward a higher rate of CR was observed with rapidly proliferating tumors. Patients with slowly proliferating primary tumors had a higher probability of 1.5-year survival than patients with rapidly proliferating tumors (83 vs 50%). The difference was even greater between patients with both primary and metastatic lesions proliferating slowly (100%) and patients with at least one (61%) or both lesions proliferating rapidly (60%). Pretreatment LI was not predictive for survival in subgroups of patients who attained CR, but it was quite predictive for survival in patients responding only partially or not at all (90 vs 32%, P = .025).