Body composition and weight gain are breast cancer risk factors that may influence prognosis. The Health, Eating, Activity, and Lifestyle Study was designed to evaluate the relations of body composition, weight history, hormones, and lifestyle factors to prognosis for women with breast cancer. In the cross-sectional analysis of this cohort study specific to 150 Hispanic and 466 non-Hispanic White women in New Mexico diagnosed between 1996 and 1999, the authors hypothesized that obesity measures are associated with baseline prognostic markers and that these associations are modified by ethnicity. Ethnic-stratified multiple logistic regression analyses showed divergent results for a tumor size of 1.0 cm or more and, to a lesser extent, positive lymph node status. Among Hispanics, the highest quartile for body mass index (29.5 vs. <22.5 kg/m2: odds ratio (OR) = 0.16, 95% confidence interval (CI): 0.03, 0.84) and for waist circumference (> or =95.0 vs. <78.5 cm: OR = 0.09, 95% CI: 0.01, 0.78) was significantly associated with a reduced tumor size. In contrast, for overweight and obese non-Hispanic White women, there was an increased association with obesity-related measures, particularly striking for the highest quartile of waist circumference (OR = 2.76, 95% CI: 1.45, 5.26). These findings suggest that Hispanics may have a different breast cancer phenotype than non-Hispanic Whites, which associates differently with body composition and weight history.