Surgical specimens of 80 ovarian tumors of borderline malignancy (OTBM) were investigated by scanning DNA cytophotometry. Diploid or euploid DNA histograms were found for 21 tumors, whereas 59 OTBM showed noneuploid or aneuploid DNA patterns. All patients were followed-up after surgery for at least 3 years (mean observation period, 6.7 years). Follow-up showed 11 cases of recurrent disease and 6 deaths. DNA findings and several other morphologic and clinical details (including patient age, histologic type and stage of disease, and extent of therapy) were correlated to the postoperative course. Statistical analyses disclosed that, of these parameters, only DNA content significantly affected prognosis. Recurrences and deaths resulting from tumor exclusively were observed among patients with noneuploid or aneuploid OTBM, whereas none of the diploid or euploid tumors recurred (P less than 0.05). DNA cytophotometry thus might be regarded as an effective complementary means to assess the prognosis of individual OTBM cases.