The pharmacological efficacy of serotonergic-acting drugs suggest that patients with obsessive–compulsive disorder (OCD) may have alterations in their cerebral serotonergic (5-HT) receptor system, and previous neuroimaging studies of OCD patients have shown abnormalities in several fronto-subcortical regions. In this study we investigated cerebral 5-HT2A receptor binding in 15 untreated OCD patients and in 15 age- and gender-matched healthy volunteers by magnetic resonance imaging and [18F]altanserin positron emission tomography (PET). Eleven of the patients were rescanned with PET after receiving treatment with a selective serotonin reuptake inhibitor (SSRI). The distribution volumes of specific tracer binding (DV3′) were calculated for 12 brain regions, and comparisons were made between: (1) healthy volunteers vs. untreated OCD patients, (2) healthy volunteers vs. treated OCD patients, and (3) OCD patients before and during treatment. When comparing the distribution volume for specific fronto-subcortical brain regions, significantly higher values were recorded in the caudate nuclei in OCD patients (DV3′: 0.24±0.14) compared to the healthy control group (DV3′: 0.15±0.13) (p<0.05, Wilcoxon matched-pairs test). This difference between groups was not present after treatment with SSRIs. There was no correlation between the severity of OCD symptoms and 5-HT2A receptor binding. An increase in 5-HT2A receptor binding is found in the caudate nuclei of untreated patients with OCD. The up-regulation in 5-HT2A receptors might be compensatory for a lack of serotonin in the feedback loop between the thalamus and orbito-frontal cortex, the caudate nuclei, and the globus pallidus.