The objective of this study was to determine whether comorbidity, or pre-existing conditions, can account for some of the disparity in survival between African-American and white breast cancer patients. A historical cohort study was conducted of 416 African-American and 838 white women diagnosed with breast cancer between 1973 and 1986, and followed through 1999 in the Kaiser Permanente Northern California Medical Care Program. Information on comorbidity, tumor characteristics and breast cancer treatment was obtained from medical records, and Surveillance, Epidemiology and End Results, Northern California Cancer Center Registry. Associations between comorbidity and survival were analyzed with multiple Cox proportional hazards regression. Over a mean follow-up of 9 years, African Americans had higher overall crude mortality than whites: 165 (39.7%) versus 279 (33.3%), respectively. When age, race, tumor characteristics and breast cancer treatment were controlled, the presence of hypertension was associated with all cause survival [hazard ratio (HR) = 1.33, 95% confidence intervals (CI) 1.07-1.67] and it accounted for 30% of racial disparity in this outcome. Hypertension-augmented Charlson Comorbidity Index was a significant predictor of survival from all causes (HR = 1.32, 95%CI 1.18-1.49), competing causes (HR = 1.52, 95%CI 1.32-1.76) and breast cancer specific causes (HR = 1.18, 95%CI 1.03-1.35). In conclusion, hypertension has prognostic significance in relation to survival disparity between African-American and white breast cancer patients. If our findings are replicated in contemporary cohorts, it may be necessary to include hypertension in the Charlson Comorbidity Index and other comorbidity measures.