Idiopathic pulmonary arterial hypertension (IPAH) is a progressive and fatal disease of the pulmonary artery resulting from currently unidentified etiology. IPAH is pathologically characterized as sustained vasoconstriction and vascular remodeling of the pulmonary artery. Phosphodiesterase type 5 (PDE5) inhibitors have been clinically used in the treatment of IPAH. Recently, we have shown that Ca2+-sensing receptor (CaSR) antagonists, or calcilytics, inhibit excessive cell proliferation of pulmonary arterial smooth muscle cells (PASMCs) from IPAH patients. In this study, the additive or synergistic effect of calcilytics on antiproliferation following PDE5 inhibition was examined in IPAH-PASMCs by MTT assay. Treatment with sildenafil blocked the excessive cell proliferation of IPAH-PASMCs in a concentration-dependent manner with an IC50 value of 16.9 μM. However, sildenafil (0.03–100 μM) did not affect the cell growth of PASMCs from normal subjects and patients with chronic thromboembolic pulmonary hypertension (CTEPH). Co-treatment with 0.3 μM NPS2143, a calcilytic, additively enhanced the antiproliferative effect induced by sildenafil (3 or 30 μM) in IPAH-PASMCs. Additionally, the inhibitory effect of calcilytics, NPS2143 or Calhex 231 (1 or 10 μM), on excessive cell proliferation of IPAH-PASMCs was synergistic increased in the presence of 1 μM sildenafil. Similar results were obtained by BrdU incorporation assay. These findings reveal that calcilytics additively/synergistically enhance the antiproliferative activity mediated by PDE5 inhibition, suggesting that a combination therapy of a PDE5 inhibitor with a calcilytic may be useful as a novel therapeutic approach for IPAH.