The purpose of our study was to assess the effect on pulmonary function of adding intravenous hydrocortisone to the standard treatment of infants with cystic fibrosis (CF) hospitalized for lower respiratory illnesses (LRI). Twenty CF infants were randomized and received 10 days of hydrocortisone (10 mg/kg/day) or placebo in addition to standard treatment with intravenous antibiotics, chest physiotherapy, and an aerosolized beta-agonist with cromolyn. Functional residual capacity (FRC) and forced expiratory flows (V'mak,FRC) were measured on admission, on Day 10 of hospitalization, and as outpatients 1-2 months following hospital discharge. Pulmonary function values were adjusted for differences in body length and expressed as Z-scores. Upon admission flows were decreased, and FRC was increased in both groups; there were no differences between the groups. The change in pulmonary function from admission to Day 10 of hospitalization was not different for the two groups. From admission to outpatient follow-up after hospitalization, there was a significant increase in flows for the steroid group, but not for the placebo group. In addition, the direction of change in FRC was significantly different for the two groups; the steroid group had a small decrease in FRC, while the placebo group had a small increase in FRC. These findings suggest that the addition of intravenous hydrocortisone to the standard treatment of CF infants hospitalized for a LRI may produce a greater or a more sustained improvement in lung function following hospitalization.