We report three cases of fatal hyperthermia in the course of neuroleptic drug treatment. One patient developed his symptoms within hours after a single drug dose whereas the two others had a prolonged symptomatology which lasted for several days after the administration of different neuroleptic drugs. Biochemical events included diffuse intravascular coagulation, muscle cell necrosis and acute renal failure. Rhabdomyolysis was proved pathologically in two patients but brain lesions were atypical in all three cases. We conclude that the biochemical and pathological abnormalities seen in these patients are those which have been described by some authors in the malignant hyperthermia syndrome after anaesthesia. The symptomatology however can be insidious and the syndrome can develop after withdrawal of the drugs.