Very few functional neuroimaging studies have been performed on patients with obsessive-compulsive disorder (OCD) undergoing behavior therapy, even though it is recognized to be an effective treatment for this disorder. We measured the regional cerebral blood flow (rCBF) using the Xenon inhalation method in 31 treatment-refractory patients with OCD and the same number of age-matched normal controls. We also studied changes in rCBF in 22 OCD patients who had demonstrated a significant improvement after the behavior therapy. The OCD patients showed a significant bilateral elevation in the rCBF in the basal ganglia compared with the normal controls. After successful treatment, a significant decrease was found in the rCBF in the right head of the caudate nucleus that tended to correlate with clinical improvement.