This study investigates differences in the effects of comorbidities on survival in Medicare beneficiaries with prostate cancer. Medicare data were used to assemble a cohort of 65- to 76-year-old Black (n = 6,402) and White (n = 47,458) men with incident localized prostate cancer in 1999 who survived >or=1 year postdiagnosis. Comorbidities were more prevalent among Blacks than among Whites. For both races, greater comorbidity was associated with decreasing survival rates; however, the effect among Blacks was smaller than in Whites. After adjusting for age, socioeconomic status, and community characteristics, the association between increasing comorbidities and survival remained weaker for Blacks than for Whites, and racial disparity in survival decreased with increasing number of comorbidities. Differential effects of comorbidities on survival were also evident when examining different classes of comorbid conditions. Adjusting for treatment had little impact on these results, despite variation in the racial difference in receipt of prostatectomy with differing comorbidity levels.