The neuroendocrine system modulates the immune response through neuropeptides and neurohormones, findings which point to the existence of a neuro-endocrine-immune system regulatory axis. At the same time, there is growing evidence that the pineal gland has anti-neoplastic properties, which include the action of its principal hormone, melatonin (MLT), on the immune system through the release of cytokines by activated T-cells and monocytes. The present study was carried out on 31 patients (19 males and 12 females, age range 46-73 years) with advanced solid tumors (7 gastric, 9 enteric, 8 renal, 5 bladder, 2 prostate) who either failed to respond to chemotherapy and radiotherapy or showed insignificant responses and were therefore shifted to MLT therapy (10 mg/die orally for 3 months). We obtained blood samples just before the start of MLT administration and after 30 days of therapy. Plasma was collected in EDTA tubes on ice, immediately centrifuged at 4 degrees C and stored frozen at -80 degrees C; samples were measured by immunoradiometric assays (Medgenix-Fleurus, Belgium) for tumor necrosis factor alpha (TNF), interleukin-1, 2 and 6 (IL-1, IL-2, IL-6) and interferon gamma (IFN). We used Student's paired t-test to compare each patient's cytokine circulating levels before and after MLT administration and found a significant differences (p < 0.05). After 3 months of therapy, none of our patients displayed adverse reactions to MLT or had to discontinue treatment. Nineteen patients (61%) showed disease progression. The other 12 (39%), however, achieved disease stabilization with no further growth of either the primary tumor or of secondaries; moreover, they experienced an improvement in their general well-being, in terms of Tchekmedyian's criteria, associated with a significative decrease of IL-6 circulating levels. These findings are consistent with the hypothesis that MLT modulates immune function in cancer patients by activating the cytokine system which exerts growth-inhibitory properties over a wide range of tumor cell types. Furthermore, by stimulating the cytotoxic activity of macrophages and monocytes, MLT plays a critical role in host defence against the progression of neoplasia.