Malignant Hyperthermia (MH), Lethal Catatonia (LC), and Neuroleptic Malignant Syndrome (NMS) are known as rare potential lethal diseases. MH and NMS--both being drug-induced--have in common some main symptoms. LC and NMS, on the other hand, are hardly distinguishable by clinical means. After presentation of the case reports the differential diagnosis of the syndromes is discussed. While there is strong evidence for the MH to be drug-induced, the NMS cannot be explained sufficiently in this way. Clinically both can be differentiated. Lethal catatonia is a syndrome rather than a specific disease. Differential diagnosis for NMS is possible by the neuroleptic withdraw-trial.