Methoxyflurane (Penthrane) has been shown to cause acute polyuric renal failure. Two patients recently manifested typical polyuric renal failure for two to three days following abdominal surgery with the use of methoxyflurane anesthesia. However, they then developed oliguric renal failure, requiring hemodialysis. Both showed x-ray film evidence of shrinkage of renal mass. The mean length of the renal silhouettes decreased from 13.5 to 11.3 cm in one patient and from 12.5 to 10.9 cm in the other. Renal biopsy in each patient disclosed intratubular and intertubular calcium oxalate crystal deposition and severe interstitial fibrosis. A second biopsy and autopsy in one patient demonstrated progression of the fibrosis. The latter patient died 14 months after surgery while undergoing chronic dialysis. The other patient no longer requires dialysis and has a serum creatinine level of 8 mg/100 ml.