Elevated plasma brain natriuretic peptide (BNP) levels have been described in patients with congestive heart failure and acute myocardial infarction. We measured plasma BNP levels in patients with chronic respiratory failure to evaluate the correlation between plasma BNP levels and pulmonary haemodynamics. Plasma BNP levels were measured in 28 patients with chronic respiratory failure accompanied by three underlying diseases [14 with chronic obstructive pulmonary disease (COPD), seven with sequelae of pulmonary tuberculosis (sequelae Tbc) and seven with diffuse panbronchiolitis (DPB)] by immunoradiometric assay methods (IRMA). Twenty-one of 28 patients had already received oxygen supplementation and 16 of 21 patients were treated as outpatients with home oxygen therapy. Plasma BNP levels were significantly elevated in patients with chronic respiratory failure complicated by cor pulmonale (81·5 ± 13·1 pg ml⁻¹) compared to patients without cor pulmonale (13·3 ± 2·7 pg ml⁻¹, P<0·001). As controls, plasma BNP levels in 10 patients with primary lung cancer were studied, and the results (3·5 ± 1·0 pg ml⁻¹) were not significantly different from those of patients with chronic respiratory failure without cor pulmonale. Plasma BNP levels in 12 healthy subjects were also studied, and the results (7·2 ± 1·0 pg ml⁻¹) were not significantly different from those of the control subjects. Plasma BNP levels showed a weak linear correlation with systolic pulmonary arterial blood pressure, estimated by Doppler echocardiography (r=0·43; P=0·068), but there was no significant correlation between BNP levels and the degree of hypoxaemia (r=0·30; P=0·138). Plasma atrial natriuretic peptide (ANP) levels in patients with chronic respiratory failure were also measured using the same samples. Plasma ANP levels were also significantly elevated in patients with chronic respiratory failure complicated by cor pulmonale (80·8 ± 12·1 pg ml⁻¹) compared to patients without cor pulmonale (26·1 ± 4·4 pg ml⁻¹, P=0·003). A significant correlation was found between plasma BNP and ANP levels (r=0·68; P<0·001). Our results suggest that the plasma BNP or ANP level may be a useful indicator for detecting the presence of cor pulmonale in patients with chronic respiratory failure.