Abnormal growth of vascular smooth muscle cells is seen in various pathological conditions such as hypertension, atherosclerosis, and restenosis. Na+/H+ exchanger (NHE) activation appears to play a permissive role in vascular smooth muscle cell proliferation and vascular remodeling. The present study investigated the effect of a new specific NHE-1 inhibitor, sabiporide, on human pulmonary artery smooth muscle cell proliferation and migration. Concentrations of sabiporide as low as 20 μmol/L in the culture medium containing growth factors inhibited cell proliferation, as measured by cell counting, and also inhibited the rate of DNA synthesis, as examined by measuring BrdU incorporation into DNA. Cell growth inhibition was not caused by cell death, as demonstrated by the measurement of intracellular lactate dehydrogenase release and by the reversibility of inhibition upon washing. By fluorescent-activated cell sorting analysis, we are the first to demonstrate that NHE-1 inhibition arrests the cell cycle progression at G0/G1 phase, suggesting that NHE activation plays a permissive role in entrance of cells into the cell cycle. Sabiporide also concentration-dependently inhibited human pulmonary artery smooth muscle cell migration. The present study showed that sabiporide inhibits vascular smooth muscle cell proliferation and migration by blocking the cell cycle progression at G0/G1 phase.