Twenty-two infants under 1 year hospitalized with lower respiratory tract disease due to RSV documented by virus isolation or indirect immunofluorescence were enrolled in a double blind placebo controlled efficacy trial of aerosolized Ribavirin. Nine patients with ten episodes of illness were classified as high-risk because of preexisting cardiac or pulmonary disease. The majority of patients in both groups presented with respiratory distress or apnea. All four patients in the treated group had bronchopulmonary dysplasia (BPD). In the placebo group, three patients had BPD and three had cardiac disease. Drug efficacy was assessed by comparing the mean duration of the following clinical characteristics between groups: tachypnea (RR > 50), days held NPO, days of hypoxemia (pO2 < 70), days requiring supplemental oxygen, and days requiring mechanical ventilation. Although trends in favor of Ribavirin were seen for all measures other than tachypnea, using the Wilcoxon rank-sum test, significant differences were seen for the two latter characteristics (2.2 days vs. 8.0 days, and 0.2 days vs. 4.5 days).