In this open study, 93 children (aged 2-12 years) with a clinical diagnosis of pharyngitis or tonsillitis caused by Streptococcus pyogenes (culture and/or ELISA test positive) were given azithromycin, as a single daily dose of 10 mg/kg (maximum 500 mg) for three days (n = 46); or erythromycin ethylsuccinate, 30-50 mg/kg daily given in three divided doses, for ten days (n = 47). Forty-four of 46 azithromycin patients, and 46 of 47 erythromycin patients, had S. pyogenes isolated at baseline and were included in the clinical and bacteriological analyses. At the end of treatment (day 10-12), 38 (86%) of the 44 azithromycin patients were considered cured, four (9%) improved, one (2%) failed and one relapsed. In the erythromycin group, 30 of 46 (65%) were considered cured, 15 (33%) improved and one (2%) failed. Eradication of S. pyogenes was achieved in 40 of 44 (91%) and 45 of 46 (98%) azithromycin and erythromycin patients, respectively. Re-occurrence of S. pyogenes, assessed 28-32 days after start of treatment, occurred in five of 37 (14%) azithromycin patients (three with clinical symptoms) and five of 39 (13%) erythromycin patients (four with clinical signs). There were no statistically significant differences in clinical or bacteriological efficacy between the two groups. Both drugs were well-tolerated, with side-effects (mainly gastrointestinal) reported in five of 46 (11%) azithromycin patients and in six of 47 (13%) erythromycin patients, one of whom withdrew from treatment. No laboratory abnormalities were observed in the azithromycin patients, but were recorded in two of 43 (5%) erythromycin patients.(ABSTRACT TRUNCATED AT 250 WORDS)