Serotonin (5-HT) has been implicated in the pathogenesis of pulmonary arterial hypertension (PAH). We hypothesized that plasma 5-HT would be elevated in PAH related to associated conditions. We performed a prospective cohort study of 21 patients with PAH undergoing initial right heart catheterization and 6 healthy controls. Platelet-free plasma 5-HT levels were similar in patients with idiopathic PAH, PAH related to associated conditions, and healthy controls. Higher 5-HT levels correlated with increased six-minute walk distance (r=0.55, p=0.04). These data suggest that plasma 5-HT levels are normal in PAH. Plasma 5-HT may therefore not be a useful biomarker in this condition.