This prospective study compares the fractional excretion of sodium, FENa, urinary sodium concentration, UNa, urine osmolality, Uosm, and the U/P creatinine ratio in their diagnostic effectiveness in 87 patients with acute renal failure: 22 acute tubular necrosis, 18 non-oliguric acute tubular necrosis, 12 acute urinary tract obstruction, 14 acute glomerulonephritis, and 21 pre-renal azotemia. Discriminant analysis demonstrated a correct diagnostic classification in 86 of 87 patients using FENa, and only 46, 60 and 65 correct using Uosm, UNa, and U/P Cr, respectively. FENa is identified as the most effective non-invasive test for the differential diagnosis of acute renal failure. An FENa of 1 classifies all entities into two groups: FENa more than 1; acute tubular necrosis, non-oliguric acute tubular necrosis and urinary tract obstruction and less than 1; pre-renal azotemia and acute glomerulonephritis (P less than 0.001).