Milk may be one of the risk factors in the development of breast cancer from epidemiological investigations. Our study investigated the hormones and main ingredients in milk and assessed the effects of milk on the development of 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary tumors in rats by comparing differences among three groups: commercial milk (C), traditional milk (T) or water (W). During the 20-weeks experiment the C and T groups showed higher incidences of mammary tumors than the W group. After excluding potential confounding factors including fat and calcium, the C group was found to score higher than the T group in the indices of tumorigenesis. These findings suggested that DMBA-induced mammary tumors are more prevalent in milk-fed groups due in part to the contribution of estrogen and progesterone in milk.