One hundred forty-two children with presumed Group A beta-hemolytic streptococcal (GABHS) pharyngitis were enrolled in a randomized double blind prospective study comparing the consequences of immediate penicillin treatment with treatment delayed for 48 to 56 hours. One hundred fourteen of the enrolled patients were culture-positive. An adverse impact of early antibiotic therapy was noted; the incidence of subsequent infections with GABHS was significantly greater in those treated at the initial office visit with penicillin. In the month following documented evaluation of GABHS, a recurrence occurred 2 times more frequently in those treated with penicillin immediately compared with those for whom treatment was delayed 48 to 56 hours. Late recurrences (beyond 1 month but in the same streptococcal season) occurred 8 times more frequently (P less than 0.035). Delay in penicillin treatment did not increase GABHS intrafamilial spread. Symptoms of both groups were assessed for 2 days following the initiation of treatment. Both placebo-treated and penicillin-treated groups used aspirin or acetaminophen ad libitum. Penicillin was shown to reduce fever and relieve sore throat, dysphagia, headache, abdominal pain, lethargy and anorexia significantly beyond that achieved with aspirin or acetaminophen alone. Penicillin had no effect on culture-negative cases.