Cytodiagnostic urinalysis was tested to determine its utility in the differential diagnosis of acute renal failure (ARF). Fifty-one patients with acute renal failure were included and evaluated clinically with regard to the etiology of the renal failure, whether underlying chronic renal failure was present, and if dialysis was required. Urine specimens were macroscopically examined and subjected to a multiparameter reagent-strip analysis. Papanicolaou stain was done on cytocentrifuge preparations and the number of blood cells, renal cells, and casts examined in a standardized fashion. The results showed that the 34 patients with acute tubular necrosis (ATN) of either ischemic or toxic origin had a higher number of collecting duct cells, and a higher total number of casts than the 17 non-ATN patients. Twelve patients requiring dialysis had a higher number of different types of casts (granular, waxy, leukocytic, broad casts) as well as more renal cells (mainly necrotic) than the 39 patients who did not require dialysis. A significant positive correlation was found between the magnitude of rise of serum creatinine and a number of cytodiagnostic parameters. We conclude that cytodiagnostic urinalysis may be valuable in addition to other tests in the evaluation of patients with acute renal failure.