In 40 out of 99 patients (40.4%) with solid tumours of different tissue, but the same stage (IV), elevated serum levels of interleukin-10 were observed. The mean levels of the cytokine in patients with malignant melanoma (24.3 ng/ml), pancreatic (6.8 ng/ml) or gastric (6.3 ng/ml) adenocarcinoma were significantly higher than in healthy subjects (3.4 ng/ml) or in patients with uterine fibroma (1.7 ng/ml). Patients with colon (6.8 ng/ml) and renal (5.7 ng/ml) carcinoma had similar values of interleukin-10 but did not significantly differ from controls. Interleukin-10 is known to suppress the functions of both T lymphocytes and macrophages, working as a general dampener of the immune and inflammatory responses. The observation of increased circulating levels of interleukin-10 in cancer patients may have important implications for future investigations, immunological monitoring and therapeutic intervention on neoplastic patients, and suggests a mechanism for tumour cells escaping from immune surveillance.