A case of neuroleptic malignant syndrome (NMS) in a 23 year old male patient is reported. The symptoms were hyperthermia, muscle rigidity, change in mental status, sinus tachycardia, creatinine phosphokinase elevation and myoglobinuria. The patient suffered from severe muscle pain and compromised respiratory function. Treatment was cessation of neuroleptic medication and institution of intensive medical care focusing on symptomatic treatment. One week after admission clinical status and laboratory findings were normalized and the patient was readmitted to a psychiatric hospital. The neuroleptic medication of the reported patient had been olanzapine during seven months at a dose of 25 mg daily. The day before onset of NMS the pharmacological treatment was supplemented by 100 mg of clozapine. The cause of onset of NMS in this case is discussed. Clozapine, an atypical neuroleptic, is known to have reduced potential to cause NMS and in such cases without extrapyramidal symptoms. Olanzapine, however, has not yet been reported to cause NMS. Alternatively the cause of onset of NMS in this patient could be explained by the combination treatment and possible synergistic effect of the two antipsychotic drugs. Further research in this field is needed.