The metastatic non-small cell lung cancer (NSCLC) still remains an untreatable disease, and the role played by chemotherapy has yet to be defined. The new immunotherapeutic strategies, such as interferon and IL-2, seem to be also less effective, since they generally determine only a stabilisation of disease. On the basis of previous experimental results suggesting a synergistic action between IL-2 and the pineal neurohormone melatonin (MLT), a study was started to evaluate the clinical efficacy and toxicity of a neuroimmunotherapeutic combination consisting of IL-2 plus MLT as a first line therapy in metastatic NSCLC. The study included 20 patients (adenocarcinoma: 10; epidermoid cell carcinoma: 7; large cell carcinoma: 3). MLT was given orally at a dose of 10 mg day-1 at 8.00 pm every day, starting 7 days before the onset of IL-2 administration. IL-2 was given subcutaneously at a dose of 3 x 10(6) IU m-2 every 12 h for 5 days/week for 4 weeks, corresponding to one cycle of immunotherapy. In responder patients or in those with stable disease, a second cycle was given after a rest-period of 21 days. A partial response was achieved in 4/20 (20%) patients. Ten other patients had a stable disease (50%), whereas the last six patients progressed. Toxicity was low in all cases. This study shows that the neuroimmunotherapeutic therapy with IL-2 and the pineal hormone MLT may represent a new effective and well tolerated treatment in metastatic NSCLC, with results comparable to those obtained with chemotherapy, but with an apparent lower biological toxicity.