Pulmonary arterial hypertension (PAH), de®ned as elevated pulmonary arterial pressure and pulmonary vascular resistance, is an end-point of a variety of conditions. The only therapy that has been shown to improve both quality of life and survival is intravenous prostacyclin (prostaglandin I 2 (PGI 2 ), epoprostenol).The effect of long-term aerosolized iloprost (Ilomedin, Schering, Berlin, Germany and Vienna, Austria), a stable prostacyclin analogue and potent vasodilator, on haemodynamics and functional status was investigated in 12 patients with severe pulmonary hypertension. Haemodynamic measurements and vasodilator testing by right heart catheterization were performed prior to and after long-term iloprost inhalation therapy.Haemodynamic improvement or increased exercise tolerance was not observed in any of the patients. After a meansd treatment period of 105 months, meansd pulmonary vascular resistance had increased from 113 Wood Units (mmHg . L -1 . min) to 134 Wood Units, with unchanged arterial oxygen saturation (924% versus 914%). Within the study period, three patients went into right heart failure and had to be placed on intravenous epoprostenol.The authors conclude that inhaled iloprost in addition to conventional therapy in the presently recommended dose of 100 mg . day -1 delivered in 8±10 2 h portions, is not an ef®cient vasodilator therapy in severe pulmonary hypertension. It remains to be shown whether dose increases and/or combination protocols will be effective, or whether inhalation of iloprost may be safe for selected cases of pulmonary hypertension. Eur Respir J 2001; 17: 8±13.