The therapeutic efficacy of ribavirin, an anti-viral agent with a broad spectrum of activity, was studied in 14 infants with bronchiolitis which, in seven cases, was caused by a respiratory syncytial virus infection. The drug was administered for 5-6 days using an aerosol in periods of either 4 or 6 h with an interval of either 4 or 2 h between administrations. The body temperature of patients with fever returned to normal by day 3 of therapy. A significant decrease in the respiratory rate occurred by day 2 when there was also a marked improvement in the subjective and objective symptomatology. The most significant clinical improvements occurred in infants presenting a respiratory syncytial virus antigen in nasal wash specimens. This suggests therapeutic selectivity by ribavirin for (or a more pronounced sensibility of) this virus.