There has been an increased incidence of malaria among Europeans returning from Africa and Asia. The relatively new antimalarial mefloquine (Lariam) has become extremely popular due to its efficacy in treating the wide-spread chloroquine-resistant Plasmodium falciparum. Mefloquine is used both for prophylaxis and treatment of malaria and is relatively well tolerated. However, since introduced in 1985, there have been over 100 reports of severe neurologic and psychiatric adverse effects associated with its use, including acute psychosis, affective disorders, acute confusional states and seizures. We describe a 39-year-old woman who developed acute psychosis after being given mefloquine prophylaxis. Adverse effects occur more often after therapeutic rather than prophylactic use, and those with a history of seizures or psychiatric illness are at increased risk of developing these reactions. Physicians should be aware of these possible side effects and prescribe mefloquine only when indicated.