This study evaluated the concurrent effects of diet (standard AIN-76A, restricted AIN-76A and high-fat diet) and moderate rotating-drum treadmill exercise on the incidence of 7,12-dimethylbenz(a)anthracene-induced mammary carcinomas in virgin female BALB/cMed mice free of murine mammary tumor virus. Exercise significantly reduced food consumption in the groups fed the standard diet and high fat diet. Although food consumption varied widely, final body weight was similar in all groups. Exercise had no effect on mammary tumor incidence in the standard diet-fed groups (without exercise, 47%; with exercise, 45%); however, exercise reduced mammary tumor incidence in the other groups as follows: restricted diet/no exercise, 28%; restricted diet/with exercise, 13%; high fat diet/no exercise, 31%; high fat diet/with exercise, 19%. Restricting food intake reduced mammary tumor incidence, but had no effect on body weight. Although energy intake was related to mammary tumor incidence, neither body weight nor dietary fat predicted tumor incidence.