We report a consensus statement of the collaborative research group on the prevention and treatment of malignant syndrome (MS) in Parkinson's disease. The syndrome is quite similar to neuroleptic MS. Although sudden withdrawal of levodopa was the most frequent cause, many other precipitating events were found such as intercurrent infections, dehydration, hot weather, discontinuation of other anti-parkinsonian drugs, and "wearing off" phenomenon. Awareness of this syndrome is most important for its early detection and the prompt commencement of treatment. MS should be suspected whenever the body temperature rises above 38 degrees C without an apparent cause. Treatment consists of ample intravenous fluid, cooling the body, anti-parkinsonian drugs (particularly levodopa and bromocriptine), dantrolene sodium, and antibiotics if infection is present. Rhabdomyolysis, disseminated intravascular coagulation, and acute renal failure constitute serious complications.