The similarities between the serotonin (5-HT) transporter in both human platelets and human brain permit us to investigate this structure in patients with different psychiatric disorders. Several reports have shown abnormalities of the 5-HT transporter, by means of the measurement of the 5-HT uptake or of the 3H-imipramine binding, in platelets of patients with obsessive-compulsive disorder (OCD). The availability of the ligand 3H-paroxetine, a selective 5-HT reuptake inhibitor, to label the 5-HT transporter, promoted us to evaluate the binding of 3H-paroxetine in platelets of 18 drug-free patients with OCD. The results, showing that the patients had a lower number of 3H-paroxetine sites, which is inversely correlated with the Yale Brown Obsessive-Compulsive Scale total score, than a similar group of controls, add supporting evidence to the involvement of 5-HT in OCD. In addition, the decreased functionality of the 5-HT transporter seems to be linked to the severity of OC symptoms.