Background: Little is known about serotonin neurons in Parkinson disease (PD). Objective: To study the serotonin system in PD with positron emission tomography, using the serotonin transporter radioligand [11C] (+)McN5652. Design and Patients: We measured the density of the serotonin transporter and the density of [11C]WIN35428-labeled dopamine transporters in the striatum of 13 adults with PD and 13 age- and sex-matched controls. To assess the effects of possible differences in blood flow or brain atrophy, we also measured regional cerebral blood flow and the size of the regions of interest for the caudate nucleus and putamen. Results: Patients with PD showed reductions in the specific distribution volumes of [11C] (+)McN5652 in the caudate (P<.01) and putamen (P<.01), along with the expected reductions in striatal [11C]WIN35428 binding (P<.01). There were no reductions in regional cerebral blood flow or the sizes of the regions of interest, mitigating against potential confounding effects of blood flow, brain atrophy, or partial volume effects. Reductions in serotonin transporter binding correlated with ratings of disease staging. Conclusions: These results suggest that the density of serotonin transporters, like that of dopamine transporters, is reduced in the striatum of patients with PD and that these changes are related to disease stage.