We assessed distributions of breast cancer prognostic biomarkers by race/ethnicity and socioeconomic position among paraffin-embedded tumor biopsy specimens from 135 US women (48 white women, 44 black women, 43 Asian women) diagnosed with breast cancer between 1966 and 1990. No racial/ethnic or socioeconomic differences in distributions were observed for tumor stage, lymph node involvement, estrogen, progesterone, and epidermal growth factor receptors, oncogenes such as Her2/neu and p53, cytoplasmic proteins cathepsin-D and ps2, and two indices of cell growth, Ki67 and DNA ploidy, adjusting for age at diagnosis, menopausal status, place of birth and, for racial/ethnic comparisons, working class composition of census block-group at diagnosis. Black and Asian women, however, were 3.5 times (95% confidence interval [CI] = 1.2, 10.1) and 3.7 times (95% CI = 1.3, 10.6), respectively more likely than white women to have a tumor size of > or = 20 mm, and Asian women were 3.4 times (95% CI = 1.1, 10.4) more likely than black women to be positive for androgen receptor, adjusting for these same factors. No differences in distributions by socioeconomic position were observed for these latter two tumor characteristics. These data suggest that racial/ethnic and socioeconomic disparities in breast cancer survival are unlikely to be explained solely by differential distributions of molecular breast cancer prognostic biomarkers.