Scant data are available on lung function in acute respiratory distress syndrome (ARDS) in pediatric patients. We measured respiratory mechanics by single-breath occlusion and maximum expiratory flow-volume curves by forced deflation in ten critically ill infants with clinical ARDS. Ten mechanically ventilated infants without lung disease served as the control group. To assess the severity of the lung injury in the infants with ARDS, we modified an adult scoring system that calculates a score (from 0 to 4; >2.5 indicates severe lung injury) based on the extent of chest radiographic changes, degree of hypoxemia, amount of positive endexpiratory pressure (PEEP), and total respiratory system compliance. The lung injury scores of our patients were in the range of 2.75 to 3.75. The lung injury scores of the control group were zero. The predominant alteration in lung function was restrictive, as characterized by a significant decrease in total respiratory system compliance (0.41 ยฑ 0.13 ml/cmH 2 O/kg versus 1.12 ยฑ 0.16 ml/cmH 2 O/kg of controls; P < 0.001) and forced vital capacity (21.5 ยฑ 6.5 ml/kg versus 59.2 ยฑ 6.3 ml/kg of controls; P < 0.001). Maximum expiratory flow rates at 10% forced vital capacity were significantly increased (23.6 ยฑ 20.1 ml/kg/sec versus 8.4 ยฑ 2.5 ml/kg/sec of controls; P < 0.05), confirming the absence of any significant obstructive abnormalities. The passive expiratory flow-volume curves were curvilinear and convex in shape, indicating inhomogeneous lung pathology. The inhomogeneous distribution of lung injury in ARDS restricts the validity of respiratory mechanics measurements that rely on a single-compartment model. However, the forced deflation technique allows accurate spirometric assessments of the severity of restrictive (and obstructive) lung function changes in intubated infants with severe ARDS. Such measurements can be incorporated into lung injury scoring systems to classify the severity of the disease process for the purpose of outcome evaluation and to evaluate the effect of therapeutic interventions.