Mixed-models analyses revealed significant main effects of group for orbital frontal (F1,47=5.64, P=.02) and amygdala (F1,47=6.60, P=.01) volumes, such that patients with OCD had reduced overall volumes of these structures. The interaction of group and hemisphere was statistically significant for amygdala volume (F1,50=4.18, P=.046) such that patients with OCD had significantly reduced right amygdala volume compared with healthy comparison subjects (t1,50=3.27, P=.002; Table 1). Other results included a significant main effect of hemisphere for anterior cingulate (F1,50=12.43, P<.001), amygdala (F1,50=12.39, P<.001), and orbital frontal (F1,44=7.94, P=.007) volumes, with larger anterior cingulate and amygdala volumes in the right hemisphere and larger orbital frontal volume in the left hemisphere in patients with OCD and healthy comparison subjects. There was also a significant main effect of sex for superior frontal gyrus (F1,47=10.0, P=.003) and amygdala (F1,47=8.92, P=.005) volumes, such that women had larger superior frontal gyrus volume and men had larger amygdala volume. None of the group×sex interactions was statistically significant for any of the brain structure volumes. The distributions of the unadjusted total orbital frontal and amygdala volumes are presented in Figure 1 and Figure 2, respectively.