Variable degrees of acute renal failure developed in three patients receiving therapy with cephalothin sodium. The course and findings were consistent with acute tubular necrosis of the oliguric and nonoliguric types. One patient had protracted oliguria, a second experienced transient oliguria, and one had normal urine output. All had urinary sediment changes consistent with tubular necrosis, and the two oliguric patients had elevated urine sodium concentrations. No other causes for renal failure could be detected, and all recovered after discontinuation of cephalothin therapy, although peritoneal dialysis was required in one patient. These observations indicate that cephalothin is capable of inducing renal damage in man.