Serum levels of IL-6 were evaluated in a large group of patients with benign or malignant gynecological tumors. The results obtained were correlated with the patients' clinicopathological features and follow-up data. Using a highly sensitive immunoenzymatic method for the evaluation of serum IL-6 levels, we observed that > 95% of normal healthy women exhibited values within the range of 1.9-6 pg/ml. Using a cut-off of 6 pg/ml, elevated levels of serum IL-6 were found in 53% of 45 patients with primary epithelial ovarian cancer and less frequently in patients with endometrial and cervical cancer (37% and 10% respectively). Elevated levels of IL-6 were occasionally seen in patients with benign disease. IL-6 serum levels appeared to be less sensitive than CA 125 in ovarian cancer diagnosis. In cancer patients, increased IL-6 serum levels were related to the presence of the tumor since all post-operative patients exhibited a marked decrease. In patients with advanced ovarian cancer post-operative levels of IL-6 correlated with residual disease. Very high levels of IL-6 were observed in the ascitic fluid of 9 ovarian cancer patients, but IL-6 mRNA was not detected in tumor cells. This suggests that the increased production of IL-6 observed in ovarian cancer is reactive. Higher levels of IL-6 were found in patients unresponsive to chemotherapy, as compared with responsive ones. Univariate analysis of survival data suggests that increased IL-6 serum levels correlate with negative prognosis.