The Malignant Neuroleptic Syndrome (MNS) is characterised by the acute appearance of hyperthermia, muscular rigidity, loss of motor control, and alterations in the level of consciousness, which could prove fatal if not rapidly diagnosed and treated. It is held to be a serious idiosyncratic reaction which appears in patients being treated with neuroleptics, independently of the dosage and the length of time the drug has been prescribed. Relapses do not usually occur when the drug is re-prescribed, once the acute phase has been passed, which suggests the existence of predisposing factors. There are frequent complications (acute respiration difficulties, acute kidney failure, disseminated intravascular coagulation and multiorganic failure) which condition the prognosis. The treatment consists of the suppression of the neuroleptic, rehydration, and specific drugs (bromocryptine, sodium dantroline). We have analysed two new cases which reacted badly, one of them with a multiorganic failure and the other, who had a good initial therapeutical response but who went on to develop a peripheral neuropathy, an infrequent complication in international medical casebooks.