Recent experiments have shown that a great variety of neurohormones can interact with IL-2 in the modulation of host antitumor immune response. On the basis of these data, a study was started to evaluate the effect of the pineal hormone melatonin (MLT) on IL-2-induced immune changes in cancer. The study included 30 advanced cancer patients. They were randomized to be treated with IL-2 at a dose of 3 million IU subcutaneously twice/daily (8.00 a.m. and 8.00 p.m.) for 6 days/week for 4 weeks, with IL-2 once/daily in the evening, with IL-2 once/daily plus MLT at 10 or at 50 mg/day. MLT was given orally at 8.00 p.m. Both IL-2 given twice daily and IL-2 given once daily and IL-2 given once daily in association with MTL induced a significant increase in mean number of lymphocytes, T lymphocytes, NK cells, CD25-positive cells and eosinophils, whereas the single administration of IL-2 alone was unable to determine a significant rise in the mean number of immune cells. Soluble IL-2 receptor and neopterin increase was significantly higher during IL-2 given twice/daily than during IL-2 plus MLT, while no difference was seen in TNF rise. This study would suggest that a single daily injection of low-dose IL-2 is able to efficiently activate the lymphocyte proliferation in cancer patients when it is given in association with the pineal hormone MLT.