Tumor characteristics of 963 newly diagnosed invasive breast cancer cases from the population-based Black/White Cancer Survival Study were evaluated. Representative slides of the tumors were requested from all participating hospitals of three metropolitan areas and reviewed by one expert pathologist, blinded in regard to the age and race of patients. Nine tumor characteristics were evaluated for black and white patients. After adjusting for age, stage, and metropolitan area, blacks were significantly more likely to have high grade nuclear atypia [odds ratio (OR) = 1.97, 95% confidence interval (CI) = 1.27-3.04]; high mitotic activity (OR = 2.05, 95% CI = 1.34-3.14), grade 3 tumors (OR = 1.58, 95% CI = 1.02-2.45), and more necrosis (OR = 1.51, 95% CI = 1.16-1.98); and less likely to have well defined tubular formation (OR = 0.57, 95% CI = 0.42-0.77), marked fibrosis (OR = 0.65, 95% CI = 0.45-0.94), and positive estrogen receptor status (OR = 0.78, 95% CI = 0.58-1.05). These black/white differences remained after controlling for socioeconomic status (SES), body mass index, use of alcohol and tobacco, reproductive experience, and health care access and utilization. No significant racial differences were found for blood vessel invasion and lymphatic invasion. Although white women of high SES had more favorable tumors than those of low SES, the same pattern was not observed for blacks. High SES black women had statistically nonsignificant elevated ORs of a high mitotic index and tumor grade. These racial differences in tumor biology may have etiological and clinical implications.