Several antipsychotic agents are known to prolong the QT interval in a dose-dependent manner. The antipsychotic drugs are substrates of the phase I of biotransformation enzymes of cytochrome P450. In order to find the possible influence of polymorphism of GSTT1 (a member of class theta glutathione S-transferase) on rate-corrected QT interval (QTc) of schizophrenia patients, the present study was done. Forty-three schizophrenia in-patients participated in the study. The patients were diagnosed as chronic schizophrenia according to structured clinical interview using SCID-I (clinician version) to confirm and document DSM-IV diagnosis. Measurements of QT and RR intervals were recorded using a magnifying grid on lead II. The QTc was calculated according to Bazett's formula. Polymerase chain reaction-based method was used in order to determine the GSTT1 genotypes. Based on the fitted model of multiple linear regression analysis, QTc decreased in persons with positive GSTT1 genotype in comparison with the null genotype (β′=′−0.328, t′=′−2.346, p′=′0.024). Active genotype of GSTT1 decreased the QTc. Also, QTc was significantly associated with smoking status; it was decreased in smokers compared with nonsmokers (β′=′−0.372, t′=′−2.372, p′=′0.014).