Twenty-two hundred sixty-two consecutive medical and surgical admissions were evaluated prospectively to determine the contribution of iatrogenic factors to the development of renal insufficiency in hospital. Of 2,216 patients at risk, some degree of renal insufficiency developed in 4.9 percent. Decreased renal perfusion, postoperative renal insufficiency, radiographic contrast media, and aminoglycosides accounted for 79 percent of the episodes. Iatrogenic factors, broadly defined, accounted for 55 percent of all episodes. Poor prognostic indicators included oliguria, urine sediment abnormalities and, most importantly, severity of renal insufficiency; with an increase in serum creatinine of 3 mg/dl or greater, the mortality rate was 64 percent. Age, admission serum creatinine levels, and the number of episodes of renal insufficiency did not significantly affect outcome. We conclude that there is a substantial risk of the development of renal failure in hospital and that the mortality rate due to hospital-acquired renal insufficiency remains high.