To the Editor: We report on a 61-year-old man with a rapidly progressing form of familial idiopathic pulmonary arterial hypertension. Five years earlier, the patient had presented with signs of right-sided heart failure in our pulmonary-hypertension referral center. During the previous year, his condition had progressively deteriorated, despite advanced combination therapy with oral bosentan (125 mg twice daily),¹ inhaled iloprost (nine inhalations per day),² and oral sildenafil (50 mg three times a day).³,⁴ The patient's six-minute walking distance had progressively declined during the previous nine months, from 323 m to 260 m, and the pulmonary vascular resistance increased from . . .