To quantify the relationship between surfactant pool size and severity of respiratory disease, 21 lambs were delivered at 134 to 136 days gestational age and ventilated by varying only peak inspiratory pressure to maintain tidal volume at 6.2 +/- 0.3 ml/kg (mean +/- SE) and thus to control PCO2. Compliance measurements were used to quantify the severity of lung disease. After alveolar wash, surfactant phosphatidylcholine, saturated phosphatidylcholine, and minimal surface tensions were estimated. Compliance correlated linearly with saturated phosphatidylcholine pool size (r = 0.755, p less than 0.001). The mean minimal surface tension of the alveolar washes was 17.4 +/- 1.7 dynes/cm, and alveolar washes from lambs with more compliant lungs had lower minimal surface tensions than did washes from lambs with poorly compliant lungs (p less than 0.001). Lung tissue of all lambs contained similar amounts of saturated and total phosphatidylcholine, and in vitro rates of incorporation of labeled choline and palmitate into phosphatidylcholine in lung slices were similar, independent of severity of lung disease. The pool size of surfactant within the alveoli is an important determinant of lung disease in premature lambs; however, surface tension, tissue maturity, and other factors may contribute to the severity of the disease.