Bacterial infections causing acute exacerbations of chronic obstructive pulmonary disease (AECOPD) frequently require antibacterial treatment. More evidence is needed to guide antibiotic choice. The Moxifloxacin in Acute Exacerbations of Chronic Bronchitis TriaL (MAESTRAL) was a multiregional, randomised, double-blind non-inferiority outpatient study. Patients were aged ≥ 60 yrs, with an Anthonisen type I exacerbation, a forced expiratory volume in 1 s < 60% predicted and two or more exacerbations in the last year. Following stratification by steroid use patients received moxifloxacin 400 mg p.o. q.d. (5 days) or amoxicillin/clavulanic acid 875/125 mg p.o. b.i.d. (7 days). The primary end-point was clinical failure 8 weeks post-therapy in the per protocol population. Moxifloxacin was noninferior to amoxicillin/clavulanic acid at the primary end-point (111 (20.6%) out of 538, versus 114 (22.0%) out of 518, respectively; 95% CI -5.89-3.83%). In patients with confirmed bacterial AECOPD, moxifloxacin led to significantly lower clinical failure rates than amoxicillin/clavulanic acid (in the intent-to-treat with pathogens, 62 (19.0%) out of 327 versus 85 (25.4%) out of 335, respectively; p=0.016). Confirmed bacterial eradication at end of therapy was associated with higher clinical cure rates at 8 weeks post-therapy overall (p=0.0014) and for moxifloxacin (p=0.003). Patients treated with oral corticosteroids had more severe disease and higher failure rates. The MAESTRAL study showed that moxifloxacin was as effective as amoxicillin/clavulanic acid in the treatment of outpatients with AECOPD. Both therapies were well tolerated.