Hispanics are more likely to be diagnosed with breast cancer at a younger age, with more advanced stage at diagnosis, hormone receptor-negative tumors, and worse prognosis than non-Hispanic whites (NHW). Little is known regarding the association between behavioral risk factors and breast tumor characteristics and whether these associations vary by race/ethnicity. We evaluated the association between several behavioral risk factors and tumor phenotype in a population-based study of Hispanics and NHWs. Participants are cases (846 Hispanic and 1,625 NHW women) diagnosed with breast cancer between 1999 and 2004 in Arizona, Colorado, New Mexico, or Utah. The association between breast cancer characteristics and obesity, physical activity, smoking, alcohol intake, and reproductive factors was examined. Logistic regression was used to compute the ethnic-specific odds ratios for the association between these risk factors and estrogen receptor (ER) status, tumor size, and histologic grade. Hispanics had more ER-negative tumors (28 vs. 20%), tumors >2 cm (39 vs. 27%), and poorly differentiated tumors (84 vs. 77%) than NHW. Among premenopausal women, obesity was associated with more ER-negative cancers among NHW [OR = 2.47 (95% CI: 1.08, 5.67)] but less ER-negative cancers among Hispanics [OR = 0.29 (0.13, 0.66)]. Obesity was associated with larger tumors among NHW [OR = 1.58 (1.09, 2.29)], but not among Hispanics. Never using mammography was associated with larger tumors in both ethnic groups. Moderate alcohol drinking and moderate and vigorous physical activity were weakly associated with smaller tumors in both ethnic groups. Our findings suggest that the association of obesity and other behavioral risk factors with breast cancer characteristics differ by ethnicity. We observed a divergent pattern between Hispanic and NHW cases in the association between obesity and ER status and tumor size. These observations suggest that a complex set of metabolic and hormonal factors related to estrogen and insulin pathways influence tumor characteristics.