Hyperplasia of pulmonary artery smooth muscle cells (PA-SMCs) is a hallmark pathological feature of pulmonary hypertension (PH). Serotonin (5-HT) is involved in the hyperplasia through its interactions with specific receptors and internalization by a specific plasma membrane transporter. We investigated the expression and role of the 5-HT transporter (5-HTT) and 5-HT_1B, 5-HT_2A, and 5-HT_2B receptors in lungs and isolated PA-SMCs from patients with primary PH (n=14), pulmonary veno-occlusive disease (n=4), or secondary PH (SPH, n=8) and nonpulmonary hypertensive control subjects. Whereas strong immunostaining for the three receptor types and 5-HTT was seen in remodeled pulmonary vessels from patients in all PH categories, only 5-HTT expression was increased in lungs and cultured PA-SMCs from patients versus controls. The increased growth response of PA-SMCs from patients with primary PH, pulmonary veno-occlusive disease, or SPH to 5-HT or serum was entirely attributable to 5-HTT overexpression, because 5-HTT inhibitors but not 5-HT receptor antagonists abolished 5-HT mitogenic activity and reduced the serum-induced growth response to similar levels in patients as in controls. The L-allelic variant of the 5-HTT gene promoter, which is associated with 5-HTT overexpression, was present homozygously in 14 of 25 (56%) lung transplantation patients with SPH but in only 27% of controls. Polymorphism of the 5-HTT gene promoter was only partly responsible for the increased 5-HTT expression in PH, because PA-SMCs from patients exhibited higher 5-HTT levels than same-genotype cells from controls and no additional promoter sequence alterations were found. We conclude that 5-HTT overexpression is a common pathogenic mechanism in various forms of PH.