It has been reported that the biosynthesis of thromboxane A2 (TXA2) is enhanced in platelets in the presence of chronic obstructive pulmonary disease (COPD), and 11-dehydro-TXB2, a urinary metabolite of thromboxane, also increases in blood. In the present study, seratrodast (CAS 112665-43-7, Bronica), a TXA2 receptor antagonist, was administered to 14 patients with chronic pulmonary emphysema in the stable phase for 8 weeks. Respiratory distress was evaluated in the attending physicians' judgments using the Hugh-Jones (H-J) classification, and also by the patients themselves using the Borg scale. Respiratory function tests, including forced vital capacity (FVC), percent of one second forced expiratory volume (FEV1.0%), arterial blood gases during respiration of room air, and peak expiratory flows (PEF) (morning and evening), and measurement of plasma 11-denhydro-TXB2 and TXB2 levels were performed before and 8 weeks after the start of administration, as well as at the time of the start of administration. The results revealed significant improvement of respiratory distress, evaluated on both the H-J classification and the Borg scale, at week 8. Although no significant changes were observed in plasma TXB2 levels, the plasma 11-dehydro-TXB2 level significantly decreased at week 8. Among the respiratory function parameters examined, only FVC was significantly improved. These results indicated that seratrodast is useful for the improvement of respiratory distress in patients with chronic pulmonary emphysema in the stable phase.