To assess the efficacy of erythromycin in treating acute bronchitis, 52 adults were enrolled in a randomized trial comparing a one-week course of erythromycin with placebo. Among smokers, no difference in outcome was noted. Among nonsmokers, trends favored more rapid resolution of key symptoms in the erythromycin group, but these trends did not generally achieve statistical significance. These results suggest a trial with a larger sample size.