We assayed thymidylate synthase (TS) activity and performed flow-cytometric DNA analyses in fresh tumor tissues from 38 patients with ovarian cancer. TS activity was closely correlated with prognosis but was not related to age, clinical stage, histologic type or lymph node metastases. Patients with TS activity higher than 2.0 pmol/g tissue had significantly worse prognoses than those with lower levels (p 20%) had worse prognoses than those with DNA diploidy or lower fractions. Results suggest that TS activity may be a good prognostic indicator in ovarian cancer.