In this double-blind, placebo-controlled study, treatment of measles patients with ribavirin resulted in shorter and less severe disease, as well as fewer complications, compared with patients in the placebo, group. Ribavirin was well tolerated. There were no side effects or changes in laboratory values that could be associated with drug-related toxicity. Since reported vaccine failures may increase as immunization levels rise, the use of a safe and effective therapeutic agent, such as ribavirin, will be necessary to treat these cases, as well as those occurring in unvaccinated individuals.