Hypoxia is an important factor in pulmonary hypertension and vascular remodeling. We sought to determine whether the growth capabilities of human pulmonary artery smooth-muscle cells (PASMCs) differ when isolated and grown under normoxic (95% ambient air, 5% CO2) and hypoxic conditions (2% O2, 93% N2, 5% CO2). Cells from explants of the same distal (< 1 mm external diameter) pulmonary artery segments (n = 6 patients) were used at passage 3 to 12, and responses to growth factors and prostacyclin analogues (cicaprost and iloprost) were determined by measuring [methyl-3H]-thymidine incorporation, cell number, and intracellular cyclic adenosine monophosphate (cAMP) levels. DNA synthesis was greater in hypoxia—compared to normoxia-derived PASMCs—following stimulation with 5 ng/mL platelet-derived growth factor-BB (9.1 ± 2.8-fold vs 4.2 ± 1.2-fold increase, p < 0.05) [mean ± SD] and 5% fetal bovine serum (15.4 ± 3.8-fold vs 4.4 ± 0.8-fold increase, p < 0.05). A similar differential response occurred with epidermal growth factor and insulin-like growth factor-1. In contrast, the inhibitory potency of prostacyclin analogues on DNA synthesis and cell proliferation was reduced in hypoxia, compared to normoxia-derived PASMCs. For example, inhibition of platelet-derived growth factor-stimulated DNA synthesis by cicaprost (1 nmol) was attenuated in hypoxic compared to normoxic cells (34 ± 2% vs 64 ± 2%, p < 0.05) and was associated with reduced intracellular cAMP levels (4.8 ± 0.4 pmol vs 16.4 ± 4.2 pmol cAMP/105 cells after 60 min; p < 0.02). The attenuated responses to cicaprost were reversed, at least in part, by co-treatment with cAMP-specific phosphodiesterase type 4 inhibitors roflumilast (100 nmol; 29.5 ± 2.6 pmol vs 28.0 ± 1.5 pmol cAMP/105 cells) and cilomilast. In conclusion, human PASMCs exhibit a proliferative phenotype when derived and grown in a hypoxic environment. Reducing cAMP hydrolysis, for example, by inhibiting phosphodiesterase type 4 activity, may represent a strategy for regulating PASMC proliferation and vascular remodeling in pulmonary hypertension.