Neuroleptic malignant syndrome, a dangerous but little-known complication of antipsychotic drugs, is often assumed to be rare. To assess the frequency of the syndrome in a large psychiatric hospital, the authors first reviewed the literature and developed operational diagnostic criteria. Using these criteria to survey nearly 500 neuroleptic-treated patients admitted during a 1-year period, they found that seven (1.4%) had experienced definite or probable neuroleptic malignant syndrome. In several cases, including one fatal case, the diagnosis of neuroleptic malignant syndrome was not immediately considered. The authors conclude that neuroleptic malignant syndrome may be more common than previously thought and may be underdiagnosed.