Renal ultrasonographic studies were done in 26 patients with acute tubular necrosis. Prominent findings were an increase in renal size, especially the anteroposterior diameter, and sharp delineation of swollen pyramids. The ratio of anteroposterior to longitudinal diameter (H/L) was correlated with serum creatinine levels and inversely correlated with urine osmolalities. It was well correlated with the recovery time of acute renal failure. The patient group having an increased H/L had a mean recovery time of 32.4 days and 11 of the 12 patients required hemodialysis (mean of 10 times). The patient group having a normal H/L had a mean recovery time of 15.5 days and 9 of the 14 required hemodialysis (mean of 3 times).