In a double-blind study, bronchiolitis associated with respiratory syncytial virus infection in 12 randomly selected patients treated with ribavirin aerosol improved more rapidly than in 14 control patients given saline aerosol (P = .044, Wilcoxon rank sum test, two-tailed). An estimated 10 mg of ribavirin per kilogram of body weight was administered in daily 12-hour treatments over a five-day period. Respiratory syncytial virus disappeared from secretions at about the same rate in treated and control patients. There was no local or systemic intolerance, and there was no evidence of hematologic or other organ toxicity in the ribavirin-treated patients.