Twenty-four patients with acute ciguatera fish poisoning were treated with intravenous mannitol, and each patient's condition improved dramatically. All exhibited marked lessening of neurologic and muscular dysfunction within minutes of the administration of mannitol. Gastrointestinal symptoms disappeared more slowly. Two patients in coma and one in shock responded within minutes, with full recovery after infusion. Although these observations were empiric and uncontrolled and the mechanism of action of mannitol in this disease is unclear, mannitol should be considered for initial use in patients with significant illness and morbidity from ciguatera fish poisoning.