Mannitol is an osmotic diuretic with properties that suit it to a variety of clinical situations. Despite the paucity of controlled studies that precisely define optimal use, a consensus based on published work and clinical experience may be reached. Mannitol can be used prophylactically, as a diagnostic aid, or as therapy for the oliguric state. The diuretic properties of mannitol are also useful in patients with refractory edema or intoxications by aspirin, barbiturates, or bromide. As an extracellular solute, the drug may ameliorate intracranial hypertension or symptoms of dialysis dysequilibrium. The renal and systemic effects of this versatile agent are discussed.