BACKGROUND: Little is known about the long-term impact of comorbidity among women with breast cancer. METHODS: We studied a prospective cohort of 2,272 women with breast cancer, who were recruited following initial breast cancer treatment. Associations of the Charlson comorbidity index (CCI) and hypertension with survival were evaluated in delayed entry Cox proportional hazards models. RESULTS: During a median follow-up of nine years, higher CCI scores were independently associated with an increased risk of death from all causes [HR, 1.32; 95% confidence interval (CI), 1.13-1.54] and from nonbreast cancer causes (HR, 1.55; 95% CI, 1.19-2.02), but not from breast cancer (HR, 1.14; 95% CI, 0.93-1.41). Hypertension was independently associated with an increased risk of death from all causes (HR, 1.55; 95% CI, 1.20-1.99), from nonbreast cancer causes (HR, 1.67; 95% CI, 1.10-2.54), and from breast cancer (HR, 1.47; 95% CI, 1.03-2.09), but these associations were no longer significant after adjustment for antihypertensive medication. The relationship between the CCI and overall survival was the strongest among women with stage I disease (stage I, HR, 1.65; 95% CI, 1.26-2.16 vs. stage III, HR, 0.53; 95% CI, 0.23-1.25). CONCLUSION: The CCI was independently associated with lower overall and nonbreast cancer survival, but not with breast cancer-specific survival. IMPACT: Comorbidity may play an important role in breast cancer outcomes.