We have described a case of reversible acute renal failure caused by acute pyelonephritis. In this entity, the kidneys are swollen by an interstitial infiltrate and edema, and white cell tubular casts and microabscesses may be present. Fractional excretion of sodium is high, and nephrotic proteinuria may occur without glomerular abnormalities. Recovery of renal function may occur if antibiotics are promptly instituted. Renal size generally decreases after recovery.