14 cases of mannitol-induced acute renal failure were reported. The dosage of mannitol used varied widely. In all cases serum Na+, HCO3- were decreased, K+ and BUN increased significantly. Serum osmolality was measured in 5 cases. The osmolal gap was increased greatly, 77.4mOsm/kg. H2O in average. The increase of osmolal gap may play an important role in acute renal failure by causing intensive renal vasocontraction. Monitoring of serum osmolality or osmolal gap can help to prevent mannitol intoxication. The decrease of serum Na+ may be a warning sign of increased osmolal gap. Hemodialysis is the best way for the treatment of mannitol-induced acute renal failure.