The effect of ribavirin aerosol on experimental respiratory syncytial virus (RSV) infection was evaluated in a double-blind controlled study of 16 young adult volunteers. Two days after intranasal inoculation with RSV, half of the subjects were treated with a small-particle aerosol of ribavirin and half with placebo for a total of 12 hours each day for three days. Seven of eight placebo-treated and six of eight ribavirin-treated subjects became infected. Viral shedding was diminished in the ribavirin-treated group. The proportion of infected volunteers still shedding virus on days 6 through 9 was significantly less than in the placebo-treated group. Ribavirin appeared to have no effect on minor upper respiratory tract signs, but systemic complaints and fever occurred significantly less often in the ribavirin-treated group. Ribavirin aerosol therapy was well tolerated and produced no significant changes in pulmonary function test results or signs of toxicity. This suggests that such therapy might be further evaluated in infants with RSV lower respiratory tract disease.