Pulmonary surfactant is required for normal lung function. Preliminary data from previous trials suggest that more than half of chronically ventilated premature infants, greater than 1 week of age, have at least one episode of surfactant dysfunction, as measured in vitro, associated with a low surfactant protein B content. We propose to enroll premature infants less than 1250 gm birthweight, between days 5 and 21 of life who are intubated, mechanically ventilated, with a respiratory decompensation, defined as a severity score (mean airway pressure x FIO2) rising from a baseline of < 1.8 to > 3.5, sustained for > 24 hours. Infants will receive two doses of Infasurf surfactant, 12-24 hours apart, at the standard dose of 3 ml/kg. Primary outcome is the change in respiratory severity score at 72 hours post surfactant treatment compared to pre-treatment. Sample size is 31 infants, study duration is 3 years, and recruitment of study patients will occur at the Hospital of the University of Pennsylvania, and Women and Children's Hospital of Buffalo.