Inhaled nitric oxide lowers pulmonary capillary pressure (PCP) in animals and in patients with acute respiratory distress syndrome (ARDS). A dose-response relationship in patients with ARDS has not yet been established. Therefore, we studied the effects of four concentrations of nitric oxide (1, 10, 20 and 40 volumes per million (vpm)) in random order, on PCP in 19 patients with ARDS. PCP was estimated by visual analysis of the pressure decay curve after balloon inflation of the pulmonary artery catheter. Haemodynamic and gas exchange variables were measured at each nitric oxide concentration. Patients were classified as responders when PCP decreased by at least 2 mm Hg after nitric oxide 20 vpm. In responders (n = 8), nitric oxide decreased PCP and post-capillary vascular resistance dose-dependently and changed longitudinal distribution of pulmonary vascular resistance with a maximum effect at 20 vpm. In non-responders (n = 11), PCP did not change. In both groups, the nitric oxide-induced decrease in pre-capillary vascular resistance was small with a maximum effect at 1 vpm. In ARDS, vasodilatation of pre-capillary vessels is achieved at low concentrations of nitric oxide, whereas the effect of nitric oxide on postcapillary vessels is variable. Higher concentrations may be required for optimal post-capillary vasodilatation in a subgroup of ARDS patients.