Living cells from the urinary tract can be examined by phase-contrast microscopy in a pellet obtained by centrifugation of 10 ml of freshly voided urine. Once these cells have been identified and classified according to their sources, their respective proportions can be evaluated, thus providing some information on the renal structures affected. Urine sediment examination was performed in 60 cases of acute renal failure in order to determine the relationship between the abnormalities encountered and the clinical or histological diagnosis. An abnormal sediment was always associated with parenchymal acute renal failure. Cellular debris and casts were abundant in acute tubular necrosis and less numerous in toxic acute renal failure than in failure resulting from shock. The finding of deformed erythrocytes was strongly suggestive of glomerular nephropathy, a diagnosis which was confirmed by renal biopsy in almost every case.