Objective: The aim of this study is to determine the thymidine labeling index and its prognostic role in patients with ovarian cancer. Methods: Tumor cell proliferation in 32 patients with primary ovarian cancer admitted to Istanbul Medical Faculty, Department of Obstetrics and Gynecology, between 1993 and 1997 was investigated using the [H-3]thymidine labeling index (TLI). TLI results were compared with other clinical and histopathologic prognostic parameters. Results: The mean and median TLI values of the patients were 9.3 +/- 6.2% and 9.20% (range: 0.4-23.0%), respectively. Sixteen patients showed high proliferation rates (mean TLI: 14.3%). These patients had an overall survival rate of 46.7% at 3 years. The mean TLI level and overall survival at 3 years in the low proliferation rate group were 4.4 and 68.8%, respectively. Patients with a high TLI had a significantly shorter survival compared to those with a low TLI (P < 0.01). There was tendency towards a higher TLI with advanced stage (P > 0.05). However, there was no statistically significant correlation between TLI and other prognostic parameters. Conclusion: TLI may have a predictive value in determining the outcome of patients with ovarian cancer. Further larger scale studies are needed before definite conclusions can be made about its role as a prognostic factor in this disease. (C) 2001 International Federation of Gynecology and Obstetrics. All rights reserved.