Our objective was to analyze detailed anthropometric characterization for risk of breast cancer in Uruguayan women. The design was a case-control study. The setting was Pereira Rossell Women's Hospital, Montevideo, Uruguay. Subjects were 343 incident breast cancer cases and 1,042 frequency-matched healthy controls who were interviewed on menstrual and reproductive story; and a series of skin folds, circumferences, and diameters were measured to calculate fat and muscle fractions and the derived fat-to-muscle ratio (FMR). Odds ratio (ORs) coefficients were taken as estimates of relative risk derived from unconditional logistic regression. Muscle fraction was negatively associated with risk [OR for highest quartile = 0.23, 95% confidence interval (CI) = 0.15-0.34], fat fraction was positively associated (OR = 3.90, 95% CI = 2.62-5.80), and FMR was positively associated (OR = 4.45, 95% CI = 2.99-6.62). Stratified analyses by body mass index levels also showed risk increases for the highest tertiles of FMR, always displaying significant linear trends. Since increases of risk were found in overweight and in normal weight women, results suggest that fractions and amount of muscle and fat components might be risk factors for breast cancer on the basis of currently existing metabolic and immune interrelationships between adipose and muscular tissue given by glutamine, exercise-derived myokines, and other cytokines produced by these tissues.