A 7-month-old male Papillon was presented to us with mild dyspnea, cyanosis and a diastolic murmur detected by cardiac auscultation. Echocardiography revealed severe pulmonary arterial hypertension (PH), and administration of 1 mg/kg of oral sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor, twice daily was initiated. Exercise capacity, cyanosis, dyspnea and cardiac murmur were improved after therapy for 4 weeks. PCV was remarkably high (74%) after therapy for 3 years, however, increasing the dose of sildenafil decreased this value (60%). Follow-up after therapy for 4 years revealed that treatment with oral sildenafil only continued to provide the dog with an excellent quality of life, without any side effects.