We undertook a prospective, double-blind, placebo-controlled trial to resolve the question of the clinical effectiveness of ribavirin in previously well infants who require ventilation for respiratory distress secondary to respiratory syncytial virus (RSV) bronchiolitis. Aerosol ribavirin or NaCl 0.9% was administered within 24 h of initiation of ventilation, 18 h/d, for a maximum of 7 d or until extubation. From March 1994 to March 1997, 42 children were randomized and 41 patients were retained for analysis. Baseline characteristics of each group-ribavirin and placebo (20:21)-were not significantly different with respect to age (62.5 +/- 35.9 versus 62.7 +/- 30.9 d), sex, weight, and length of ventilation pre-aerosol. "Intent to treat" outcome analysis found no significant differences in the length of the following: ventilation (102.16 +/- 65.26 versus 126.28 +/- 78.72 h; p = 0.29), aerosol therapy, stay in the intensive care unit, total oxygen therapy, and hospitalization. The aerosols were well tolerated and no deaths occurred. This trial demonstrates the lack of effectiveness of aerosolized ribavirin in reducing the length of ventilation and course of illness in infants with no underlying illness ventilated for respiratory distress secondary to RSV bronchiolitis.