Infection with respiratory syncytial virus (RSV) is a common cause of respiratory disease in infancy. Surfactant phospholipids have been shown to be reduced in severe RSV infection. Reduction in surfactant proteins might also contribute to the pathogenesis of this disease. We investigated daily levels of surfactant proteins in bronchoalveolar lavage (BAL) fluid from 18 ventilated infants with RSV infection (median age 3.1 mo) and in a control group of 16 ventilated surgical patients (median age 0.4 mo). Surfactant proteins were measured by ELISA, total protein by the Lowry method. Surfactant protein A (SP-A) was reduced in BAL fluid from children with RSV infection (median 5.6 µ g/ml; range 0.6 to 151.9 µ g/ml) compared with control samples (median 9.0 µ g/ml; range 0.5 to 139.6 µ g/ml, p = 0.0368). Surfactant protein B (SP-B) was lower in the RSV group (median 12.0 ng/ml; range 0 to 60.8 ng/ml) than in control patients (median 118.1 ng/ml; range 0 to 778.2 ng/ml, p < 0.0000). Surfactant protein D (SP-D) was also reduced in the RSV group (median 130.3 ng/ml; range 0 to 1,486.0 ng/ml) versus median 600.4 ng/ml; range 0 to 1,869.0 ng/ml, p < 0.0000. Total protein levels were higher in the RSV group (median 0.49 mg/ml; range 0.13 to 2.46 mg/ml versus median 0.36 mg/ml; range 0.07 to 1.65 mg/ml, p = 0.0079). The median value of SP-A was significantly lower in the initial sample (2.3 µ g/ml) than in the final one (6.0 µ g/ml). However, no significant correlation was found between surfactant protein concentrations and disease severity measured by arterial alveolar oxygen ratio. We conclude that alterations in surfactant protein concentrations are present in severe RSV infection and speculate that these may contribute to the abnormalities of lung function seen in this condition.