Pulmonary arterial hypertension (PAH) secondary to pulmonary Langerhans cell histiocytosis (PLCH) is known to be a relatively common complication and is associated with a poor prognosis. However, the optimal therapeutic approach for these cases remains to be established. A 57-year-old man visited our hospital because of a progressive dry cough. A thoracic computed tomography examination showed a combination of diffuse thick-walled cysts and reticulonodular shadows that were predominant in bilateral upper lobes of the lungs. He was diagnosed as having PLCH based on the results of video-assisted thoracoscopic lung biopsies. During a 3-year clinical course, his condition deteriorated despite smoking cessation. A systemic evaluation demonstrated precapillary PAH caused by PLCH (PAH-PLCH), and treatment with tadalafil, a phosphodiesterase-5 inhibitor, was started. During a 50-month period of treatment with tadalafil, improvements in his dyspnea, 6-min walking distance, and hemodynamics were maintained without either overt hypoxemia or pulmonary edema. We considered that tadalafil therapy may be a useful option in the treatment of patients with PAH-PLCH.