Numerous studies have demonstrated differences in certain biological breast cancer characteristics associated with survival, including hormone receptor status and histology, among women of different racial and ethnic groups. However, women classified as "Asian or Pacific Islanders" or "Hispanic whites" represent heterogeneous populations, and few studies have separately evaluated subgroups of these populations with respect to these breast tumor characteristics. Using data obtained from 11 cancer registries that participate in the Surveillance, Epidemiology, and End Results (SEER) Program, the tumor characteristics of 93,317 women in whom invasive breast cancer was diagnosed from 1992 to 1998 were compared by race and ethnicity using unconditional and polytomous logistic regression. The study consisted of 75,978 non-Hispanic whites, 6,915 African Americans, 203 Native Americans, 5,750 Asians/Pacific Islanders, and 4,471 Hispanic whites. Eight Asian/Pacific Islander and four Hispanic white subgroups were also analyzed separately. Relative to non-Hispanic whites, African Americans, Native Americans, Filipinos, Chinese, Koreans, Vietnamese, Indians/Pakistanis, Mexicans, South/Central Americans, and Puerto Ricans living in the United States had 1.4- to 3.1-fold elevated risks of presenting with estrogen receptor-negative/progesterone receptor-negative breast cancer. Numerous differences by histological type, including lobular, ductal/lobular, mucinous, comedocarcinoma, tubular, and medullary histologies, were also observed by race/ethnicity. Breast cancer tumor characteristics differ by race/ethnicity in the United States. Both biological and lifestyle factors likely contribute to these findings. Our results may explain, to some extent, the differences in breast cancer stage and survival observed by race/ethnicity. Understanding the factors underlying these differences may provide further insight into breast cancer etiology in different populations.