Background: Dopaminergic availability is known to linearly decline in Parkinson's disease (PD). In contrast, temporal characteristics of serotonergic markers like the serotonin transporter (SERT) in relation to clinical staging of PD and dopaminergic cell loss are less clear. This study investigated SERT availability using [ ¹²³I]-ADAM and single-photon emission tomography (SPECT) in drug-naive, de novo patients, i.e., in a PD stage where dopaminergic decline starts to lead to the occurence of the characteristic motor symptoms. Methods: Nine de novo patients with PD and 9 age-matched healthy controls were studied. Measurements were repeated after 3months of levodopa treatment in patients with PD, and dopaminergic transporter (DAT) binding was examined at baseline using [ ¹²³I]-FP-CIT SPECT. Results: No alterations of SERT availability were found between groups, and neither correlation between SERT and DAT nor effects of levodopa treatment on SERT was found in patients with PD. Conclusions: These preliminary findings indicate that midbrain SERT is preserved in unmedicated patients at this early stage of PD, supporting the view that serotonergic decline temporally follows dopaminergic cell loss.