Disparities in breast cancer survival have been observed between African American and white women. There are also known differences in mean baseline white blood cell (WBC) count among racial and ethnic groups. If the WBC count falls below conventionally defined treatment thresholds for patients undergoing adjuvant chemotherapy, reduced doses or treatment delays may occur, which could lead to race-based differences in treatment duration. We used the tumor registry at Columbia-Presbyterian Medical Center to identify 1178 women with newly diagnosed stage I and II breast cancer from whom we collected base-line information for 73 African American women and 126 age- and tumor stage-matched white women. Of these women, 43 African American and 93 white women underwent adjuvant chemotherapy. African American women had statistically significantly lower WBC counts than white women at diagnosis (6.2 x 10(9)/L for African American women versus 7.4 x 10(9)/L for white women, difference = 1.2, 95% confidence interval [CI] = 0.2 to 1.2; P =.02) and after treatment (5.3 x 10(9)/L for African American women versus 6.4 x 10(9)/L for white women, difference = 1.1, 95% CI = 0.2 to 2.5; P =.03). Overall, African American women required a statistically significantly longer duration of treatment than white women (19 weeks versus 15 weeks, respectively, difference = 4 weeks, 95% CI = 0.5 to 7.2 weeks; P =.03). The lower baseline WBC counts and longer duration of treatment for early-stage breast cancer in African American women compared with those in white women result in lower dose intensity of treatment for African American women, possibly contributing to observed racial differences in breast cancer survival.