Neuroleptic malignant syndrome (NMS) is a potentially fatal adverse reaction to psychopharmacologic treatment. Reported herein are two NMS patients with schizophrenia who were found to possess a CYP2D6 gene deletion allele (CYP2D6*5). The deletion results in decreased CYP2D6 activity, possibly leading to drug accumulation. Both patients with NMS had been treated with neuroleptics, including CYP2D6 substrates. Polymerase chain reaction (PCR) followed by restriction fragment length polymorphism analyses and long PCR were performed to detect CYP2D6 genotype. One patient was found to possess *5/*10; the other had a *1/*5 genotype. The present preliminary report suggests that pharmacokinetic factors cannot be excluded and the CYP2D6 polymorphism is possibly associated with the etiology of NMS.