Thirty patients were admitted to the Anti-Poison Center of Marseille, France, on the fifth day of ciguatera fish poisoning due to ingestion of the same barracuda in Mexico. The purpose of this prospective study conducted during management of this group was to identify prognostic factors and evaluate the efficacy of treatment using mannitol. Upon admission severity was individually scored based on clinical findings. Twenty-two patients exhibiting the highest severity scores were immediately treated by infusion of 250 ml of 20% mannitol within one hour. These patients presented low plasma cholinesterase levels. Treatment led to transient improvement mainly with regard to cramping and digestive manifestations. All patients who presented high severity scores upon admission reported persistent manifestations lasting between 1 and 7 months. In 6 of these patients symptoms recurred after ingestion of seafood and/or alcohol. The fact that the extent of clinical symptoms in terms of severity and duration was correlated with the amount of fish ingested confirms that ciguatera poisoning is dose-dependent in man. However cholinesterase levels were not correlated with the severity of clinical manifestations.