21 untreated ovarian cancer patients with stage III and minimal tumour size, were given weekly intraperitoneal (i.p.) carboplatin (150 mg/m2) and alpha-2b interferon (IFN) (30 million U/m2) for a total of 12 courses, from June 1989 to February 1993. To date, a total of 248 courses have been administered. Toxicity was seldom severe, although fever (179 courses), fatigue (141 courses) and other IFN-related side-effects were very frequent. No patient refused to continue treatment, but in 5 patients IFN dose had to be reduced, and in 1 it was discontinued. The IFN mean delivered dose intensity was 19.8 million U/m2 week. Grade 3-4 myelotoxicity occurred in 7 patients (39 courses), but no deaths related to treatment occurred. The actual mean dose intensity of carboplatin was 121.5 mg/m2 week. To date, 20 patients have completed treatment and are evaluable for response. Of 11 patients with tumour size < or = 5 mm, 10 (91%) achieved a pathological complete response (pCR) as did 4/9 (44%) of those with tumour > 5 mm at entry, for a 70% (95% confidence interval 50-90) overall pCR rate. At a median follow-up of 21 months (range 4-46), only one death occurred. The probability of being alive at almost 4 years was 91% in the entire group (100% in those with tumour size less than 5 mm). Only 1 of 14 patients who achieved a pCR relapsed. This i.p. combination seems a feasible approach to previously untreated ovarian cancer patients with minimal tumour burden. IFN dosage should be reduced to improve tolerance. In view of the very high pCR rate achieved in the group of patients with smaller tumours, a randomised trial is warranted to compare this approach to standard treatment in these patients.