Our preliminary studies in humans have shown that the pineal neurohormone melatonin (MLT) may enhance the antitumor activity of IL-2, by confirming the existence of a neuroendocrine control on cytokine effects. On this basis, a study was started to evaluate the influence of a concomitant administration of MLT and low-dose IL-2 in cancer patients, who had progressed during a previous immunotherapy with IL-2 alone. The study included 14 patients with advanced solid tumors (lung 6; kidney 4; stomach 2; liver 1; melanoma 1). IL-2 was given at a daily dose of 3 million IU s.c. for 6 days/week for 4 weeks. MLT was given orally at a daily dose of 40 mg every day, starting 7 days prior to IL-2. Objective tumor regression, consisting of a partial remission (PR), was achieved in 3/14 (21%) patients (lung 1; kidney 1; liver 1). Six other patients had a stable disease (SD), while the remaining 5 cases progressed. PR and SD were associated either with a significantly longer survival at 1 year, or with a significantly higher increase in lymphocyte and eosinophil mean number with respect to the patients with disease progression. This preliminary study suggests that advanced solid neoplasms resistant to IL-2 may become responsive to IL-2 therapy by a concomitant administration of the pineal hormone MLT, which could act by enhancing IL-2 antitumor immune effects and/or by increasing the susceptibility of cancer cells to the cytolysis mediated by IL-2-induced cytotoxic lymphocytes.