Ciguatera is a distressing, hitherto-untreatable and not rare disease which results from the eating of ciguatoxin-contaminated fish from tropical and subtropical waters. We report here the results of a pilot study to assess the efficacy of mannitol therapy in ciguatera poisoning. Twelve adult patients (six men) have been treated, five of whom--who were ill acutely--experienced a significant benefit from this therapy, in three cases, with a hitherto-unexperienced dramatic reversal of symptoms. We conclude that an intravenous infusion of 1.0 g/kg of mannitol which is given over 45 minutes, after rehydration if required, can be of significant benefit to at least some acutely intoxicated victims. We postulate either a reduction of axonal oedema, or a scavenger effect, or both, as the mechanism of the beneficial effects of mannitol. Ciguatoxin is rich in hydroxyl groups, and causes microscopic oedema of neural tissue. If our conclusion of the beneficial effects of mannitol therapy is confirmed, this will offer the first effective therapy for acute phases of this disease, and has promise of preventing much long-term morbidity.