In the United States, Blacks have poorer survival rates than Whites for breast cancer. The root of this difference--social or genetic--is unclear. Utilizing the Western Washington Cancer Surveillance System and 1980 Census block group data, we examined social class and race as predictors of breast cancer survival in 1,506 women during their first 11 years following diagnosis (251 Blacks, 1,255 Whites). In a Cox regression model, after adjustment for Black-White differences in age, stage, and histology, Black mortality was 1.35 times that of Whites (95%CI = 1.05-1.72). Following additional adjustment for social class, as measured by a variety of block group characteristics, Black mortality was only 1.10 times that of Whites (95%CI = 0.83-1.46). In both Blacks and Whites, poorer social class was a powerful determinant of shortened survival. These results indicate that the observed breast cancer survival differences between Black and White women today in the US today is substantially due to the poorer social class standing of Blacks.