We have extracted from the literature data from 100 animal experiments, involving 7838 rats and mice, which compared the effects of different levels of dietary fat and/or calorie intake on the development of mammary tumors. Both higher calorie intake (P less than 0.0001) and higher fat intake (P less than 0.0001) independently increased mammary tumor incidence in Sprague-Dawley rats and in mice, as judged from analyses combining ad libitum feeding experiments and restricted feeding experiments. The effect of fat was two thirds the magnitude of the calorie effect in both Sprague-Dawley rats and mice. In ad libitum feeding experiments, a modest but significant (P less than 0.0001) average increase in body weight was found in animals fed high fat diets. However, these differences in body weight did not correspond to differences in mammary tumor incidence. The effect of log body weight on the log odds of tumor incidence was not significant (P = 0.16), while dietary fat intake significantly increased tumor incidence (P less than 0.0001). The collection of animal experimental data supports the hypothesis that, in mammary tumor development, there is a specific enhancing effect of dietary fat, as well as a general enhancing effect of calories.