We read with interest the report by Hachulla et al1 (November 2009) that early-onset pulmonary arterial hypertension (PAH) was frequent in patients with systemic sclerosis and was associated with severe hemodynamic abnormalities. Most of the patients had a low carbon-monoxide diffusing capacity (Dlco), and a Dlco < 35% predicted indicated a poorer prognosis. Using sensitive indicator dilution techniques in patients with systemic sclerosis, we have demonstrated the presence of pulmonary endotheliopathy and reduced functional pulmonary capillary surface area early in the disease, even before the onset of PAH.2 Although it had been postulated for some time that decreased Dlco in the absence of interstitial fibrosis was related to reduced capillary surface area available for gas diffusion and represented the presence of PAH, this relationship had not previously been directly measured. We have recently provided the first, to our knowledge, functional confirmation that decreased Dlco relates to decreased functional capillary surface area in connective tissue disease.3 Based on our data, the prognostic threshold of Dlco <35% predicted found by Hachulla and colleagues1 should represent a profound reduction in functional capillary surface area, in the range of 25% of normal.3,4 This degree of abnormality would certainly contribute to a poor prognosis. Regular screening as proposed by Hachulla and colleagues represents the only hope for early detection and possibly better therapeutic success.