Background: The serotonin system has repeatedly been associated with the pathogenesis of pulmonary hypertension (PH). Objective: To comparatively analyze plasmatic and intrathrombocytic serotonin levels in arterial and mixed venous blood of patients with PH and unaffected controls to elucidate pulmonary serotonin metabolisms. Patients and Methods: Catheters were placed in the radial and pulmonary artery in patients with PH (n = 13) for diagnosis and in age-matched controls (n = 6) undergoing percutaneous closure of the patent foramen ovale. Arterial and mixed venous blood samples were immediately centrifuged to obtain plasma and platelets and thereafter frozen at -20 degrees C. After careful thawing, plasmatic and platelet serotonin levels were determined by ELISA. Results: PH was classified as arterial in 4 and chronic thromboembolic in 9 patients with a mean pulmonary artery pressure of 37 (interquartile range: 32-43) mm Hg. Platelet serotonin content was significantly lower in the PH patients than in the controls. The mean transpulmonary gradient (arterial-mixed venous) was negative in the PH group and positive in the controls. An inverse correlation was found between the arterial blood platelet serotonin content and pulmonary hemodynamics. Plasmatic serotonin levels did not differ between the PH and control groups. Conclusion: The lower platelet serotonin concentration in PH patients compared with unaffected controls is an unprecedented finding. The negative transpulmonary platelet serotonin gradient and the strong negative correlation of arterial blood platelet serotonin with pulmonary hemodynamics might indicate increased serotonin uptake in the lungs of PH patients.