BACKGROUND: Obsessive-compulsive disorder (OCD) is a chronic anxiety disorder of unknown aetiology. Psychopharmacological studies have suggested a role for the neurotransmitter serotonin however further evidence for serotonin in the aetiology of OCD is conflicted. The authors used positron emission tomography (PET) to examine the binding of the dopamine D(1) receptor antagonist [(11)C]-SCH23390 to D(1) receptors in the striatum of drug-free OCD patients compared with healthy controls. METHODS: Seven drug-free patients (two drug naïve) with OCD and seven age, gender and education matched healthy controls underwent positron emission tomography with [(11)C]-SCH23390. Binding Potentials (BP) at D(1) receptors were calculated for the caudate nucleus and putamen. Correlations between BP values for basal ganglia regions and clinical measures were performed in OCD patients. RESULTS: The BP for [(11)C]-SCH23390 at D(1) receptors in OCD patients was significantly reduced in both caudate nucleus (0.59+/-0.06 vs 0.88+/-0.06, p<0.05) and putamen (0.89+/-0.06 vs 1.14+/-0.06, p<0.05) compared with healthy controls. No correlations were found between D(1) BP and symptom measures. LIMITATIONS: The main limitations of this study are the small sample size and the PET methodology which does not allow for disaggregation of Bmax and Kd values for D(1) receptor binding of [(11)C]-SCH23390. CONCLUSIONS: The finding of downregulation of D(1) receptors in the striatum of OCD patients suggests increased nigrostriatal dopaminergic drive in OCD. If confirmed, this finding provides support for trials of novel treatments in OCD based on dopaminergic system blockade.