The pineal hormone melatonin (MLT) is able to exert an oncostatic action. Its possible use in the treatment of human tumors, however, has not yet been investigated. The present study was carried out to evaluate the effects of MLT in patients with metastatic solid tumors resistant to conventional therapies. The study included 54 patients, most of them were affected by lung cancer or colorectal carcinoma. MLT was given intramuscularly at a daily dose of 20 mg at 3.00 p.m. for 2 months; this induction phase was followed by a maintenance period at a dose of 10 mg orally in responder patients or in those with an improvement in performance status (PS). The clinical response was as follows: 1 partial response (cancer of pancreas), 2 minor responses (colon cancer and hepatocarcinoma) and 21 with stable disease. The remaining 30 patients rapidly progressed within the first 2 months of therapy. An evident improvement in PS was achieved in 18 of 54 (33%) cases. These results, by showing an apparent control of the neoplastic growth and an improvement in the quality of life in a reasonable number of cancer patients for whom no other standard therapy is available, would justify further clinical trials to better define the impact of MLT therapy on the survival and quality of life of untreatable advanced cancer patients.