Intraperitoneal treatment with interferon-gamma (IFN-gamma) has been shown to achieve surgically documented responses in the second-line therapy of ovarian cancer. To assess its efficacy in the first-line therapy, we conducted a randomized controlled trial with 148 patients who had undergone primary surgery for FIGO stage Ic-Illc ovarian cancer. In the control arm women received 100 mg/m(-2) cisplatin and 600 mg/m(-2) cyclophosphamide, the experimental arm included the above regimen with IFN-gamma 0.1 mg subcutaneously on days 1, 3, 5, 15, 17 and 19 of each 28-day cycle. Progression-free survival at 3 years was improved from 38% in controls to 51% in the treatment group corresponding to median times to progression of 17 and 48 months (P= 0.031, relative risk of progression 0.48, confidence interval 0.28-0.82). Three-year overall survival was 58% and 74% accordingly (n.s., median not yet reached). Complete clinical responses were observed in 68% with IFN-gamma versus 56% in controls (n.s.). Toxicity was comparable in both groups except for a mild flu-like syndrome, experienced by most patients after administration of IFN-gamma. Thus, with acceptable toxicity, the inclusion of IFN-gamm in the first-line chemotherapy of ovarian cancer yielded a benefit in prolonging progression-free survival.