Black men with prostate cancer have poorer disease-specific and overall survival rates than do their U.S. White counterparts. Comorbidity at the time of diagnosis has been shown to predict both overall survival and cause-specific mortality among White men with localized prostate cancer, but the role that initial levels of comorbidity play in determining intergroup variation in survival of prostate cancer patients is not well established. This study evaluated the effect of comorbidity at diagnosis on racial differences in survival among men with prostate cancer. Clinical and demographic data were abstracted from records of 864 patients diagnosed at four Chicago area hospitals between 1986 and 1990. Comorbidity was scored on the basis of clinical information in the Charlson index. Cause-specific relative mortality adjusted for age, stage, differentiation, and treatment was compared across Charlson scores with Cox proportional hazards functions. Blacks had significantly greater mortality from prostate cancer and other causes versus Whites. However, differences disappeared as initial comorbidity increased. The study concludes that absence of a significant preexisting medical diagnosis is associated with a higher risk for excess mortality among Black men diagnosed with prostate cancer. To the researchers' knowledge there is no biological explanation for this finding. They suggest the possibility that early-stage diagnosis may correlate with a prior history of care or pattern of early diagnosis and with secondary prevention that favorably affects patient longevity. Results may be limited due to geographic constraints of the study cohort, sample sizes and new advances in prostate-specific treatment that may have altered current management in clinically important ways.