Several lines of evidence have indicated that free radicals may play a role in the pathophysiology of tardive dyskinesia (TD) (reviewed in Andreassen and Jorgensen, 2000). NAD(P)H: quinone oxidoreductase (NQO1) is an important enzyme in the human body that counteracts the oxidative stress-induced neuronal injury caused by the toxic free radicals such as dopamine-semiquinones. Taking the possible genetic predisposition to TD into account (Yassa and Ananth, 1981), the NQO1 gene is a good candidate gene that may confer increased susceptibility to TD. Based on this hypothesis, Pae et al. (2004) reported a significant association between the Pro187Ser polymorphism in the NQO1 gene and TD. In the present study, we attempted to replicate the findings of Pae et al. (2004) with the same polymorphism in 222 Japanese patients with schizophrenia. No significant difference was detected between patients with and without TD in the allelic distribution (chi2 = 0.070, d.f. = 1, p = 0.795) and in the genotypic distribution (chi2 = 0.910, d.f. = 2, p = 0.657). In addition, there was no significant difference in terms of total Abnormal Involuntary Movement Scale scores among the three genotype groups (p = 0.49). Our results suggest that the NQO1 gene polymorphism does not confer an increased risk of TD.