The azithromycin immediate-release formulation (AZ-IR) provides effective treatment for group A beta-haemolytic streptococcal pharyngitis in adults. Single-dose therapy with a novel azithromycin extended-release (AZ-ER) formulation could reduce treatment failure and eliminate non-compliance contributing to antimicrobial resistance. A randomized, double-blind, double-dummy, multicentre trial was conducted comparing AZ-ER (single oral 2-g dose) with AZ-IR (3 days, 500 mg once daily) for the treatment of group A beta-haemolytic streptococcal pharyngitis/tonsillitis in adults and adolescents (n = 598). The primary endpoint was bacteriological eradication at test -of-cure (TOC; day 24-28) in the bacteriological per-protocol population (n = 420). Bacteriological eradication was achieved in 85.4% (175/205) and 81.4% (175/215) of subjects in the AZ-ER and AZ-IR groups, respectively (95% CI -3.1-11.1). Clinical cure at TOC occurred in 99.0% of subjects in the AZ-ER group and in 96.7% in the AZ-IR group. At long-term follow-up, bacteriological recurrence was observed in 5.5% (9/163) and 7.7% (12/156), respectively. Both treatments were well tolerated; and most adverse events (AEs) were mild to moderate in intensity. The most frequent treatment-related AE was diarrhoea, or loose stools, in 11% of both treatment groups. AZ-ER-treated and AZ-IR-treated subjects had AE burdens (AE days/patient-year) of 7.6 days and 9.2 days, respectively. A similar trend in favour of AZ-ER was noted for treatment-related diarrhoea burden (1.9 days vs. 2.5 days). A single 2-g dose of AZ-ER is as effective and well tolerated as 3 days of AZ-IR (500 mg once daily) for treating group A beta-haemolytic streptococcal pharyngitis/tonsillitis in adults and adolescents.