In 55 patients with either the oliguric and nonoliguric form of acute renal failure, some laboratory parameters for the analysis of prerenal and intrinsic types of acute renal failure were examined. The parameters were analyzed within 7 days of the clinically known beginning of the illness. The parameters were analyzed as follows: sodium in urine, creatinine urine/plasma ratio, urine osmolality, osmolality urine/plasma ratio, renal failure index, and fractional excretion of filtered sodium. Hemodialysis was performed in 29 of the 55 patients. The oliguric form of acute renal failure was present in 49 of the 55 patients. In relation to renal failure index, prerenal acute renal failure was present in 7 patients and intrinsic acute renal failure in 48. It appears that in patients with a clinical diagnosis of prerenal acute renal failure, the urinary parameters do not separate them from those with acute tubular necrosis. It also appears that in patients with laboratory diagnosis of prerenal acute renal failure (i.e., a RFT less than 1.0), the response to treatment is unpredictable and in fact may have a worse prognosis than in those with a RFI greater than 1.0 (5/7 deaths vs 10/48 deaths).