Despite the platelet production in response to IL-2, cancer immunotherapy with IL-2 tends to induce thrombocytopenia, which probably depends on an enhanced peripheral destruction. On the basis of our previous studies, this effect may be neutralized by a concomitant administration of the pineal hormone melatonin (MLT). This study was performed to investigate the influence of an immunotherapeutic combination with low-dose IL-2 and MLT on platelet number in advanced cancer patients showing persistent thrombocytopenia. The study included 14 advanced solid tumor patients, affected by thrombocytopenia due to different causes (portal hypertension: 9; previous chemotherapies: 3; DIC: 2). IL-2 was injected at 3 million IU/day subcutaneously for 6 days/week for 4 weeks, in association with MLT (40 mg/day orally). A normalization of platelet number occurred in 10/14 (71%) patients, and platelet mean number significantly increased on treatment. No important therapy-related toxicity was observed. This preliminary study would suggest that the concomitant administration of MLT is able not only to neutralize IL-2-induced thrombocytopenia, but also to increase platelet number in thrombocytopenic cancer patients.