Background: Social anxiety disorder (SAD) is associated with amygdala hyperresponsivity and imbalances in serotonergic neurotransmission. We have previously noted altered uptake of carbon-11 labelled 5-hydroxytryptophan (5-HTP) in a small sample of patients with SAD, suggesting deficiencies in presynaptic serotonin synthesis. In the present study, positron emission tomography (PET) was used to assess uptake of [11C]5-HTP in a larger sample of patients with SAD compared with age and sex-matched healthy controls. Methods: PET-data were available for 17 patients (8 females, age 33±8 years) diagnosed with SAD and for 17 healthy controls (9 females, age 35±10 years). Accumulation of the [11C]5-HTP tracer was assessed at Uppsala Imanet during 60 minutes in the resting state. Parametric images were generated using the graphical reference Patlak method assuming irreversible trapping of [11C]5-HTP from 11-60 minutes. Cerebellum was selected as reference region after correction for the decarboxylation rate of [11C]5-HTP. Exploratory and amygdala focused analyses were performed using statistical parametric mapping (SPM2). Results: Patients with SAD had significantly higher [11C]5-HTP uptake than controls in several regions including the superior, medial and inferior frontal gyrus, anterior cingulate cortex, hippocampus and lentiform nucleus, all in the left hemisphere. Region of interest analyses also revealed significantly higher uptake (SAD > controls) in the left (x-28 y-4 z-12; T=3.16) and right (x24 y1 z-15; T=2.82) amygdala (p<0.05 corrected). Conclusions: Higher [11C]5-HTP uptake, suggesting an elevated serotonin synthesis rate, was noted in patients with SAD compared to healthy controls predominantly in frontal and temporal regions including the amygdala.