The long-acting beraprost preparation TRK-100STP is formulated to provide sustained release of an orally active prostacyclin derivative to maintain the optimal plasma concentration for a longer period of time compared with the currently used conventional beraprost sodium. In the present study, we evaluated the efficacy of this newly developed formulation for pulmonary arterial hypertension (PAH).An open-label, 12-week multicenter clinical trial was performed in 46 patients with PAH. They were initially treated with 120 microg of TRK-100STP divided into 60 microg twice daily, followed by a stepwise increase to 360 microg given as 180 microg twice daily. The 6-minute walking distance showed a significant increase by 33.4+/-66.0 m (95% confidence interval [CI], 13.4 to 53.5) from the baseline measurement. Mean pulmonary artery pressure, total pulmonary vascular resistance, and pulmonary vascular resistance decreased by -2.8+/-5.5 mmHg (95% CI, -4.6 to -1.0), by -0.92+/-2.63 mmHg*L(-1)*min (95% CI, -1.78 to -0.05), and by -0.89+/-2.81 mmHg*L(-1)*min (95% CI, -1.84 to 0.06), respectively, from the baseline measurements. A higher efficacy was observed in patients with a maximum tolerated dose of 360 microg daily than those of 240 microg daily or less.Treatment with TRK-100STP for a 12-week period improved the exercise capacity, mean pulmonary artery pressure, and total pulmonary vascular resistance. TRK-100STP was effective for Japanese patients with PAH.