Acute renal failure (ARF) may be due to obstructive uropathy or renal parenchymal disease. Twenty-five children with acute renal failure secondary to renal parenchymal disease underwent ultrasonographic examination of the kidneys. Changes of renal size and cortical echogenicity were correlated with renal function. All patients presented with bilaterally enlarged kidneys with the exception of those in the neonatal age group (12%). Improvement in renal function resulted in normalization of renal size. With regard to cortical echogenicity two groups were formed. Group A comprised 11 patients whose kidneys had the same echogenicity as the liver, while in group B the kidneys were more echogenic (14 patients). Cortical echogenicity was always increased. Determination of creatinine levels showed a statistically significant difference between group A (3.32 mg% +/- 1.40 S.D.) and group B (5.95 mg% +/- 1.96 S.D.), p less than 0.001. Changes in renal function were paralleled by rapid changes in renal size and cortical echogenicity.