Sodium dodecylsulfate acrylamide gel electrophoresis (SDS-AGE) separates the proteins according to their molecular weight (MW). It was carried out on the urine of 17 healthy control subjects and 92 ambulatory patients with suspected or known renal impairment. The SDS-AGE patterns were classified as physiological, low MW predominance, middle MW predominance, high MW predominance, or mixed low and high MW. Patients were separately classified as having either normal kidneys or glomerular, tubular, or mixed renal lesions according to the results of clinical investigation. Comparison of both classifications revealed that SDS-AGE allowed a good forecast of the site of renal involvement. Predominantly low MW protein excretion correlated with tubular damage. Middle and high MW patterns correlated with glomerular disease. SDS-AGE was compared to chromatography on Sephadex G 100, to acetate cellulose electrophoresis, immunoelectrophoresis and electrophoresis in acrylamide without SDS. It was found that SDS-AGE gave the most information that was in agreement with the patient's clinical status. SDS-AGE is recommended for routine use for clinical diagnosis.