Brain uptake of [¹⁸F]FDOPA, measured with PET, reflects the activity of aromatic amino acid decarboxylase, an enzyme largely expressed in monoaminergic nerve terminals. This enzyme catalyzes a number of decarboxylation reactions including conversion of l-dopa into dopamine and 5-hydroxytryptophan into serotonin. For more than 20years [¹⁸F]FDOPA PET has been used to assess dopaminergic nigrostriatal dysfunction in patients with Parkinson's disease (PD). More recently, however, [¹⁸F]FDOPA PET has also been employed as a marker of serotoninergic and noradrenergic function in PD patients. In this study, we provide further evidence in support of the view that [¹⁸F]FDOPA PET can be used to evaluate the distribution and the function of serotoninergic systems in the brain. Eighteen patients with PD were investigated with both [¹⁸F]FDOPA and [¹¹C]DASB PET, the latter being a marker of serotonin transport (SERT) availability. We then assessed the relationship between measurements of the two tracers within brain serotoninergic structures. [¹⁸F]FDOPA uptake in the median raphe nuclei complex of PD patients was significantly correlated with SERT availability in the same structure. Trends towards significant correlations between [¹⁸F]FDOPA Ki values and [¹¹C]DASB binding values were also observed in the hypothalamus and the anterior cingulate cortex, suggesting a serotoninergic contribution to [¹⁸F]FDOPA uptake in these regions. Conversely, no correlations were found in brain structures with mixed dopaminergic, serotoninergic and noradrenergic innervations, or with predominant dopaminergic innervation. These findings provide evidence that [¹⁸F]FDOPA PET represents a valid marker of raphe serotoninergic function in PD and supports previous studies where [¹⁸F]FDOPA PET has been used to assess serotoninergic function in PD.