My colleagues and I recently reported that two mitochondrial DNA polymorphisms, 5178 A/C (2) and 10398 G/A (3), were associated with bipolar disorder. The 5178 site is not polymorphic in Caucasians because most of them have the 5178C genotype. In these positions, the 5178C and 10398A genotypes were more frequently seen in patients with bipolar disorder than in comparison subjects in our study group. In Japanese people, the 10398A genotype was found in 33% (44 of 133) of the subjects with bipolar disorder and in 22% (38 of 171) of the comparison subjects (odds ratio=1.7, p<0.05). The finding by Dr. McMahon and colleagues that the odds ratio of the 10398A genotype was higher than 2 (78% in bipolar subjects and 64% in comparison subjects) coincides with our results. Since these two independent studies among different ethnic groups showed similar tendencies, the 10398 polymorphism in mitochondrial DNA may be one risk factor for bipolar disorder, although its effect is small.