The effect of isosorbide dinitrate (ID) on the development of pulmonary hypertension (PH) has been examined in 18 patients with idiopathic diffuse interstitial lung fibrosis (IDILF) during two years of treatment. All patients responded favourably to acutely administered ID by a decrease in pulmonary vascular resistance (PVR) by 38 +/- 13% and of the pulmonary arterial pressure (PAP) by 35 +/- 11%. Two years ID therapy led to sustained haemodynamic improvement in patients in whom blood gases were stable PAP was reduced from 38 +/- 5 to 30 +/- 6 mmHg and PVR from 483 +/- 197 to 364 +/- 175 dyne s cm-5. No haemodynamic changes were noted in the subjects with progressive hypoxaemia whereas steady deterioration of central haemodynamics was recorded in those patients who died during the follow-up. It is concluded that some patients with IDILF and PH may benefit from long-term ID treatment, but that the outcome of this treatment cannot be reliably predicted from the results of the initial first ID administration.