Objective We examined sex differences in the effect of olanzapine (OLZ), risperidone (RIS), aripiprazole (ARP), or quetiapine (QTP) on mean corrected QT (QTc) intervals among 222 patients with schizophrenia. Methods Subjects were patients with schizophrenia who were treated with either OLZ (n=69), RIS (n=60), ARP (n=62), or QTP (n=31). Electrocardiographic measurements were conducted, and the QT interval was corrected using Bazett's correction formula. Results The mean QTc interval of the QTP group was significantly longer than that of the RIS group (p=0.002) or ARP group (p=0.029). The mean QTc interval of the OLZ group was also significantly longer than that of the RIS group (p=0.006). In female participants, the difference in the mean QTc interval among the four second-generation antipsychotic (SGA) groups was statistically significant (p=0.002), whereas in male patients, there was no significant difference in the mean QTc interval among the four SGA groups. Post hoc analyses showed that sex differences in QTc interval were observed only in OLZ treatment group (p=0.007). Conclusion To our knowledge, this is the first study to demonstrate sex differences in the effect of four SGAs on the QTc interval. Copyright (c) 2013 John Wiley & Sons, Ltd.