To investigate the clinical usefulness of the amino-terminal propeptide of type III procollagen (PIIINP) as an indicator of ovarian cancer behavior, 30 patients with advanced epithelial malignancy were monitored with serial serum PIIINP and CA-125 determinations before and during treatment. Initially, PIIINP and CA-125 concentrations were each separately increased in 87% of the cases and, simultaneously, in 77% of the cases. In monitoring treatment responses, PIIINP and CA-125 were identical in 17 patients (57%), both being good predictors of the clinical behavior of the disease in 16 cases and poor predictors in one case. In 13 patients (43%) they were complementary to each other. In three cases PIIINP alone and in one case CA-125 alone were clinically useful prognosis indicators. During the period of complete clinical response to cytotoxic chemotherapy of 16 patients, the CA-125 concentrations decreased to normal before the clinical disappearance of the tumor in eight cases. PIIINP did so in only two cases, thus correlating more precisely with the presence of malignancy. In second-look laparotomies, PIIINP concentrations correlated with the presence of occult cancer better than those of CA-125. In predicting recurrent malignancy in patients with transient complete response, PIIINP and CA-125 were clinically equal. According to the present data, PIIINP concentrations often give information not obtainable by CA-125, thus being useful in monitoring the clinical behavior of ovarian cancer.