Calorie restriction can inhibit or delay carcinogenesis, reportedly due to a reduction in calorie intake rather than by concurrent changes in body mass and/or composition. Our objective was to test the hypothesis that body mass and/or composition have an important effect, independent of energy intake, on the benefits or hazards associated with calorie restriction or overeating, respectively. In the first experiment, transgenic mice that spontaneously develop prostate cancer [transgenic adenocarcinoma of mouse prostate (TRAMP)] were housed at 27°C or 22°C and pair fed the same diet for 21 weeks (95% of ad libitum intake at 27°C). In the second experiment, TRAMP mice were housed at 27°C or 22°C and fed the same diet ad libitum for 21 weeks. Despite a similar calorie intake, pair-fed mice at 27°C (PF27) were heavier (28.3 ± 3.3 versus 17.6 ± 1.6 g at 21 weeks; P < 0.001; mean ± SD) and had greater fat (6.4 ± 2.1 versus 1.9 ± 0.3 g; P < 0.001) and lean mass (P < 0.001) than pair-fed mice at 22°C. Furthermore, PF27 mice had greater levels of serum leptin (P < 0.001), lower levels of adiponectin (P < 0.05), and a greater frequency of prostatic adenocarcinoma (P < 0.05). In contrast, ad libitum-fed mice housed at 22°C consumed ∼30% more calories than ad libitum-fed mice at 27°C, but there was no difference between groups in body composition or cancer progression. These results imply that the ability of calorie restriction to inhibit or delay cancer incidence and progression is mediated in part by changes in energy balance, body mass, and/or body composition rather than calorie intake per se, suggesting that excess calorie retention, rather than consumption, confers cancer risk. ©2007 American Association for Cancer Research.