A prospective study of infants under 1 y of age, ventilated for severe viral bronchiolitis, was carried out in four paediatric intensive care units in order to study surfactant activity and composition in this condition. Lung lavage fluid from 24 infants with bronchiolitis, 19 with bronchiolitis and sepsis or cardiac failure and 12 controls were analysed by the "click test" for surfactant activity and for phospholipids. Surfactant activity was present in all controls, but in only 2 of the 24 infants with bronchiolitis alone. The presence of phosphatidylglycerol correlated perfectly with the click test, suggesting that reduced activity is due to changes in surfactant lipid composition. In those with bronchiolitis plus coexisting disease, surfactant activity and phosphatidylglycerol were absent in only half. Surfactant activity and phosphatidylglycerol re-appeared by extubation. Severe viral bronchiolitis is associated with an absence of surfactant activity and PG, which resolves by clinical recovery. Infants with coexisting conditions are not always surfactant deficient. Surfactant administration is likely to be beneficial, but requires a selective approach.