Abstract: The present study was designed to investigate the effects of added dietary calcium and vitamin D on the development of epithelial cell hyperproliferation induced in the mammary gland of mice by a high-fat diet. Four-week-old C57BL/6J mice were randomly divided into three groups, and given control diet (AIN-76A), high-fat diet with reduced calcium and vitamin D (Experimental diet i) and high-fat diet with addition of calcium and vitamin D (Experimental diet I ), respectively. Nine weeks after the dietary intervention, the animals were implanted with osmotic pumps for 72 hours to infuse bromodeoxyuridine (BrdU). Findings revealed that mice on Experimental diet I had significantly increased (P M 0. 05) BrdU--labeling indices of the terminal ducts of the mammary gland, compared to control diet group i however, BrdU--labeling indices of the terminal ducts were similar (P 0. 05) in the control diet and Experimental diet Ⅱgroups. The present study further demonstrates that a high--fat diet induces hyperproliferation of epithelial cells in the terminal ducts, and also suggests that this effect of the high-fat diet can be prevented by increasing dietary calcium and vitamin D.