Summary We compared the efficacy and safety of moxifloxacin and levofloxacin for the treatment of patients with acute exacerbations of chronic bronchitis (AECB) using a prospective, randomized, double blind, parallel-group clinical trial design. A total of 563 patients with AECB were enrolled (437 efficacy-valid) at 34 centers in Mexico, Argentina, Brazil, Colombia, and Peru. Patients were randomized to oral therapy with either moxifloxacin 400 mg once daily for 5 days or levofloxacin 500 mg once daily for 7 days. Clinical success was achieved in 201 out of 221 (91.0%) patients in the moxifloxacin group, and in 203 out of 216 (94.0%) in the levofloxacin group, indicating that moxifloxacin is equivalently effective to levofloxacin. Bacteriologic eradication or presumed eradication was also similar in the two treatment groups: 92.8% in the moxifloxacin group and 93.8% in the levofloxacin group. Nausea was the most common drug-related adverse event in each treatment group. The rate of discontinuation because of adverse events was very low (⩽2%). In conclusion, a 5-day course of moxifloxacin is clinically and bacteriologically equivalent to a 7-day course of levofloxacin in the treatment of patients with AECB. The short treatment duration with moxifloxacin may have compliance advantages over other currently used therapies in the ‘real-life’ clinical setting.