Interleukin-6 (IL-6) was measured with an enzyme-immunoassay in blood samples drawn at diagnosis from 37 patients with endometrial cancer, 36 with cervical cancer, 9 with cervical intraepithelial neoplasia (CIN) and 68 with benign uterine disease. The minimal detectable dose of IL-6 was 3 pg/mL. Detectable serum IL-6 levels were found in 9% of patients with benign uterine diseases, 11% of patients with CIN, 44% of patients with cervical cancer and 11% of patients with endometrial cancer. As regards cervical cancer, serum IL-6 levels > 3 pg/mL were found in 36.0% of 25 patients with stage Ib-IIa disease and in 64% of 11 patients with stage IIb-IV disease. As regards endometrial cancer, serum detectable IL-6 levels were observed in 0% of 30 patients with stage I-II disease and in 57% of 7 patients with stage III-IV disease (p = 0.0005). These preliminary data suggest that IL-6 may be involved in the progression of uterine malignancies.