To the Editors: Pulmonary Langerhans’ cell histiocytosis (LCH)/histiocytosis X is a rare smoking-related interstitial lung disease that predominantly affects adults aged 20–40 yrs. It is commonly associated with irreversible airflow obstruction and highly variable clinical outcomes. The disease may regress spontaneously or following smoking cessation; however, in 20–30% of cases it progresses to chronic respiratory insufficiency. Despite encouraging results in case series and reports 1, no established efficacy on disease outcomes has been demonstrated with corticosteroids, vinblastine or cladribine. Although rare in patients with early disease, severe precapillary pulmonary hypertension (PH; group 5 in the clinical classification of PH) 2 is frequent in patients with advanced pulmonary LCH. Precapillary PH was present in all 21 consecutive pulmonary LCH patients 3, and in 92% of 39 patients referred for lung transplantation 4 in two series. Lung transplantation is considered the therapy of choice for end-stage pulmonary LCH and chronic respiratory insufficiency, especially among patients who exhibit severe PH 4. Whether therapy for PH may affect long-term outcomes of disease and impact timing of transplantation is currently unknown. Here, we report the long-term improvement of LCH-associated PH in a patient treated using the dual endothelin receptor antagonist bosentan, obviating the need for lung transplantation. A male aged 25 yrs was referred to our centre in 1987 with pulmonary LCH. He had smoked for <3 yrs and discontinued smoking when aged 24 yrs. Diagnosis of LCH was made at 22 yrs, with central nervous system (diabetes insipidus, hypogonadism), pulmonary (recurrent bilateral pneumothoraces) and biopsy-proven cutaneous involvement. Pleurodesis with lung biopsy was performed by open surgical parietal pleurectomy. …