We evaluated the effect of aerosolized ribavirin therapy in infants with acute bronchiolitis who admitted to SoonChunHyang University hospital from december 1992 to february 1994 . Twenty - eight infants were enrolled to this study(RSV antibody in 18 cases were confirmed by indirect immunofluorescent technique and 10 cases were clinically diagnosed as acute bronchiolitis) . Twelve infants received aerosolized ribavirin therapy and 16 infants were treated with only conservative treatment . The mean ages were 4.5+2.5 months in rebavirin group and 4.6 ± 3.1 months in control group. The results were as follows 1) In ribavirin group , the severity of illness improved more rapidly than control group . At admission , the score of severity of illness was 13.9 in the ribavirin group and 12.4 in the control group. 2) Among eight clinical variables, 6 clinical variables (nasal discharge , chest retraction , poor feeding, stridor , rales , cough ) improved more rapidly in the ribavirin group than that in control group significantly , 3) Mean respiration rate decreases more rapidly in ribavirin group than that in control group. 4) Mean heart rate decreases more rapidly in the ribavirin group than that in control group, 5) Capillary oxygen saturation improved after 1st day in the ribavirin group, but that improved after 2nd day in the control group. 6) Mean duration of treatment was 3.5 ± 0.6 days in ribavirin group and that was 5.3 1.2 days in control group, 7) There was no evidence of abnormal hematologic or chemical finding in the ribavirin group , but minor rash on eyelid was noted in 1 case. Other side effects or direct toxicity to the lower respiratory tract were not associated with the aerosol therapy .