Thrombocytopenia is a frequent complication of cancer and constitutes an absolute contraindication for chemotherapy. Recent studies have demonstrated that platelet generation may be influenced by both cytokines and neurohormones. In particular, the pineal indole melatonin has been proven to enhance platelet number in patients with thrombocytopenia due to different reasons. On this basis, we have evaluated the effects of a concomitant administration of melatonin in thrombocytopenic cancer patients undergoing chemotherapy. The study was performed in 14 metastatic breast cancer women treated by weekly epirubicin. Each cycle consisted of epirubicin at 25 mg/m2 i.v. at weekly intervals. Melatonin was given orally at 20 mg/day in the evening every day, starting 7 days prior to chemotherapy. Patients were considered as evaluable when they received at least four cycles of chemotherapy. Evaluable patients were 12/14. The induction phase with melatonin induced a normalization of platelet number in 9/12 evaluable patients, and no further platelet decline occurred in chemotherapy. Objective tumor regression was achieved in 5/12 (41%) patients. This preliminary study would suggest that melatonin may be effective in the treatment of cancer-related thrombocytopenia and to prevent chemotherapy-induced platelet decline. Until now, melatonin therapy of cancer has been generally considered as an alternative treatment to chemotherapy. In contrast, this study would suggest that melatonin may contribute to the realization of chemotherapy in metastatic cancer patients unable to tolerate the chemotherapeutic approach because of persistent thrombocytopenia.