Pimobendan, an oral inotropic drug with phosphodiesterase III-inhibitory activity, induces cAMP-dependent relaxation of vascular smooth muscle in the pulmonary artery, as well as in the systemic cardiovascular system. We report here a patient with severe primary pulmonary hypertension (PPH), who had developed right heart failure (New York Heart Association functional class IV) despite uptitrated intravenous epoprostenol, and who was treated with extremely low-dose (0.625-1.25 mg daily) pimobendan as an adjunct to prostacyclin therapy. The combination therapy of low-dose pimobendan, prostacyclin, intravenous epoprostenol and oral beraprost has been continued for over 2 years without occurrence of fatal arrhythmia, and her six-minute walk test has exceeded 400 m. We suggest that low-dose pimobendan may enhance the hemodynamic effect of prostacyclin in severe PPH.