Although data reported in several epidemiological investigations indicate that reduced consumption of dietary fat and increased levels of physical activity are associated with reduced risk for breast cancer, the results of some studies do not support these observations. Underlying this situation is the unanswered question about whether degree of body fatness, which is affected by dietary composition, total caloric intake, and energy expenditure, is the critical determinant affecting breast cancer risk. The objective of this work was to establish whether increasing energy expenditure by exercise would reduce the occurrence of mammary carcinomas induced by 7,12-dimethylbenz[a]anthracene (DMBA) in animals consuming a high fat diet to the level of occurrence observed in sedentary animals consuming a low fat diet. Female Sprague-Dawley rats were obtained at 21 days of age and maintained on a 5% (w/w) corn oil diet (AIN-76A) until they were 64 days of age. At 50 days of age, rats received either 5 mg DMBA or the solvent in which the carcinogen was dissolved. Fourteen days after DMBA intubation they were randomized into one of three groups: 5% fat (w/w), sedentary; 24.6% fat (w/w), sedentary; or 24.6% fat (w/w), exercised. Animals were exercised on a motor-driven treadmill at a belt speed of 20 m/min and a 1-degree incline for 15 min/day, 5 days/week for 18 weeks. Feeding a high fat versus a low fat diet increased the number of breast cancers induced and the rate at which they appeared in agreement with previous investigations. However, rather than retarding the development of tumors as was hypothesized, moderate treadmill exercise increased the incidence and number of cancers induced and shortened cancer latency in comparison to animals that received either the high fat or low fat diet and were sedentary. Body composition was not altered by the exercise regime imposed, although these animals weighed more than either sedentary group. These data document a heretofore unreported effect of a moderate level of aerobic work on breast cancer induction.