Eighteen patients affected with epithelial ovarian cancer have been treated by intraperitoneal alpha 2a-Interferon. The usual single dosage was 54 x 10(6) U at weekly intervals for four times (eight times in responders): 10 histologically confirmed complete remissions and 3 partial remissions have been obtained; the toxicity was represented by fever in the majority of the cases, by abdominal discomfort in some cases, and by a mild transitory neurological complication in 1 case. Seventeen of 18 patients were pretreated, and 11 of 18 had only cytologic and/or microscopic disease. alpha 2a-Interferon has proven to be very effective and moderately toxic in microscopic peritoneal disease of ovarian epithelial cancer previously treated with extensive surgery and intravenous cytotoxic chemotherapy.