This review summarizes the most relevant data relating to the potential role of melatonin (pineal secretory product) as an adjuvant therapy of tumors. Results of clinical studies were preceded by a description of experiments conducted on tumor cell lines and on laboratory animals. Most of the reports unequivocally confirmed the antioxidative and immunostimulatory action of the pineal secretory product in both in vitro and in vivo experiments. Results of studies on cell lines of various tumors showed that the anti-proliferative effect of melatonin might involve a receptor-mediated mechanism. In experiments on animals, the cardio-, nephro-, and myelo-protective action of melatonin was confirmed in the course of application of various cytostatic drugs. A meta-analysis of clinical studies in which melatonin was applied as an adjuvant drug in the therapy of various tumors pointed to some effects of its administration. Therefore, the use of melatonin could offer hope in future antitumor therapy.