We examined the effect of antibiotic therapy on the clinical course of group A beta-hemolytic streptococcal (GABHS) pharyngitis in 260 children. After a throat culture had been obtained, each child was evaluated for the presence of predetermined signs and symptoms, and was then randomized in a double-blind manner to receive penicillin V, cefadroxil, or placebo. Of the 194 children with throat cultures positive for GABHS, 68 received penicillin V, 70 received cefadroxil, and 56 received placebo. Approximately 18 to 24 hours later, each patient returned for reevaluation. Significantly fewer children who had received either penicillin or cefadroxil had persistence of each of the three objective signs and each of the three subjective symptoms than did children who had received placebo. In addition, the evaluating physician, parents, and patients all believed that significantly fewer of the patients given antibiotic failed to demonstrate overall clinical improvement.