Over 50 years have passed since the detection of melatonin in the cattle pineal gland. Initially melatonin was believed to be only a regulator of physiological circadian rhythm processes, however, further studies have revealed other characteristics of this hormone, including its anticarcinogenic activity. The aim of this work was to review the literature on biological role of melatonin, with special reference to its oncostatic activity. The review covered the articles published from the early 1960's to 2010, collected in the MEDLINE database. The majority of experimental in vitro and in vivo studies suggest that melatonin inhibits initiation and growth of hormone-dependent tumors by decreasing both the expression of estrogen receptors and aromatase activity. The protective oncostatic activity of melatonin is likely to be expressed through the inhibition of cancer cell proliferation, decrease in oxidative stress and increase in immune system activity. Although most of the studies confirm a hypothesis of an anticarcinogenic effect of melatonin, it is not clear whether these effects occur in natural conditions. The majority of experimental models have been applied in extreme conditions, including high doses of melatonin, pinealectomy or exposure to carcinogens. To date, there are only limited epidemiological data supporting experimental observations.