Estrogens plays an essential role in the etiology of human breast, ovarian and endometrial carcinoma. Activation of estrogen receptors contributes to the growth and metastasis of these common estrogen-dependent tumors found in women. Recent studies reveal that melatonin, main hormone secreted by pineal gland, has been involved in the regulation of cancer cells growth. The oncostatic properties of this molecule has been considered on different kinds of tumors but especially on hormone dependent breast cancer.