Black women with breast cancer are less likely than white women to be diagnosed while their disease is still at a localized stage. Racial differences in the prevalence of obesity in the United States have also been documented. This study was undertaken to determine the extent to which the observed racial difference in stage at diagnosis of breast cancer could be explained by racial differences in obesity, specifically severe obesity. This was a population-based, retrospective study of 145 black women and 177 white women in Connecticut who were diagnosed with breast cancer between January 1987 and March 1989. Severe obesity was associated with both race and stage at diagnosis: Black women were significantly more likely than white women to be severely obese (26% vs. 7%, respectively), and severe obesity was significantly associated with diagnosis at TNM stage II or greater (multivariate-adjusted odds ratio = 3.10, 95% confidence interval (CI) 1.28-7.52). Adjustment for severe obesity in a logistic regression model reduced the risk of later stage at diagnosis in blacks relative to whites by 33%, from an odds ratio of 1.98 (95% CI 1.22-3.19) to one of 1.66 (95% CI 1.01-2.73). The higher prevalence of severe obesity among black women may play an important role in explaining their relative disadvantage in stage at diagnosis of breast cancer.