A study of the magnetic resonance imaging (MRI) appearances of the kidneys in six normal volunteers and 52 patients is reported. Corticomedullary differentiation was seen with the inversion-recovery (IR 1400/400) sequence in the normal volunteers and in patients with functioning transplanted kidneys and acute tubular necrosis. Partial or total loss of corticomedullary differentiation was seen in glomerulonephritis, acute and chronic renal failure, renal artery stenosis, and transplant rejection. The T1 of the kidneys was increased in glomerulonephritis with nephrotic syndrome, but the T1 was within the normal range for renal medulla in glomerulonephritis without nephrotic syndrome, renal artery stenosis, and chronic renal failure. A large staghorn calculus was demonstrated with MRI, but small calculi were not seen. Fluid within the hydronephrosis, simple renal cysts, and polycystic kidneys displayed very low signal intensity and long T1 values. Evidence of recent hemorrhage into cysts was seen in polycystic kidneys. Tumors displayed varied appearances. Hypernephromas were shown to be hypo- or hyperintense with the renal medulla on the IR 1400/400 sequence. After intravenous injection of gadolinium-DTPA, there was marked decrease in the tumor T1.