Inhaled beta-adrenergic agonists can decrease arterial oxygen tension (PaO2). A decrease in PaO2 can also be observed with supine posture. We assessed the effect on PaO2 of supine posture following salbutamol nebulization inpatients with chronic airflow obstruction. Twelve patients (10 males, 2 females mean age 60 yr) with FEV1/FVC = 0.65 (mean 0.54) and resting PaO2 = 60 mm Hg while breathing room air (mean 76.5 mm Hg) were included in a prospective randomized double-blind double crossover study conducted for 2 consecutive days. On each study day, each patient successively underwent baseline PaO2 measurement; nebulization of either salbutamol (0.5% solution, 1 cm3) or saline; two supervised 30-min periods in the supine or seated posture, with PaO2 measurement at the end of each period. PaO2 values were compared with analysis of variance for repetitive measurements (6 values for each patient). PaO2 values (mean +/- SD; mm Hg) were similar at baseline (day 1: 76.5 +/- 8.1; day 2: 76.6 +/- 8.6), after saline + seated (74.8 +/- 7.2), after saline + supine (74.9 +/- 7.5), and after salbutamol + seated (74.3 +/- 7.3). In contrast, PaO2 after salbutamol + supine (70.8 +/- 10.1) was significantly lower than all other values (p <0.05), six patients exhibited large PaO2 decreases (from 9 to 21 mm Hg). Hypoxemia induced by the combination of nebulized salbutamol and supine posture may represent a potential hazard of bronchodilator therapy.