OBJECTIVE To determine if the high mortality in acute renal failure is explained by underlying illnesses (comorbidity). DESIGN Cohort analytic study. SETTING An 826-bed general hospital providing primary, secondary, and tertiary care. PATIENTS From 16,248 inpatients undergoing radiocontrast procedures between 1987 and 1989, we identified 183 index subjects who developed contrast media-associated renal failure (defined as an increase in serum creatinine level of at least 25%, to at least 177 micromol/L [2 mg/dL], within 2 days of receiving contrast material) and 174 paired subjects, matched for age and baseline serum creatinine level, who underwent similar contrast procedures without developing renal failure. MAIN OUTCOME MEASURE Death during hospitalization. RESULTS The mortality rate in subjects without renal failure was 7%, compared with 34% in the corresponding index subjects with renal failure (odds ratio, 6.5; P<.001). After adjusting for differences in comorbidity, renal failure was associated with an odds ratio of dying of 5.5. Subjects who died after developing renal failure had complicated clinical courses characterized by sepsis, bleeding, delirium, and respiratory failure; most of these complications developed after the onset of renal failure. Deaths from renal causes were rare. CONCLUSIONS The high mortality rate in acute renal failure is not explained by the underlying conditions alone. Renal failure appears to increase the risk of developing severe nonrenal complications that lead to death and should not be regarded as a treatable complication of serious illness.