Despite myriad improvements in the care of hospitalized patients, a decline in renal function remains a common event. Renal function in 4,622 consecutive patients admitted to the medical and surgical services of an urban tertiary care hospital was followed up prospectively from the time of admission. Some degree of renal insufficiency developed in 7.2% of patients. Decreased renal perfusion, medications, surgery, and radiographic contrast media were the most common causes of hospital-acquired renal insufficiency (HARI). The overall mortality rate was 19.4% and was similar among patients for all causes of renal insufficiency, except sepsis. For patients with a greater than 3.0-mg/dL increase in serum creatinine level, the mortality rate was 37.8%. As shown by previous investigators, age and preexisting renal insufficiency were risk factors for HARI. Women and blacks had less hospital-acquired renal failure. The increasing acuity of hospital admissions has been accompanied by a greater incidence of acute renal insufficiency in patients admitted to hospitals. There is a trend toward better survival in patients with a severe deterioration in renal function.