We investigated the availability of brain serotonin transporters in 10 drug-free patients with obsessive-compulsive disorder (OCD) and age-matched healthy controls in vivo using single-photon emission computed tomography (SPECT) and the radioligand [(123)I]-2beta-carbomethoxy-3beta-(4-idiophenyl)-tropane ([(123)I]beta-CIT). For quantification of regional serotonin transporter a ratio of specific to non-specific [(123)I]beta-CIT-binding was used. The availability of serotonin transporter was calculated using regions of interests (ROI) for thalamus/hypothalamus, midbrain, brainstem (highest density of serotonin transporter) and cerebellum as a reference. The mean specific to non-specific [(123)I]beta-CIT binding ratios in the thalamic/hypothalamic ROI were 4.95 +/- 0.57 (OCD patients), and 5.48 +/- 0.87 (control group). The mean ratios in the midbrain ROI were 3.51 +/- 0.45 (OCD patients) and 4.89 +/- 1.23 (controls) and in the brainstem ROI the ratios were 2.38 +/- 0.76 (OCD patients) and 3.53 +/- 1.01 (controls). This in vivo finding of significant reduced serotonin transporter availability in midbrain/brainstem using [(123)I] beta-CIT SPECT further supports the serotonin deficit hypothesis of OCD.