This randomized, double-blind, noninferiority study was designed to demonstrate that a single 2.0-g oral dose of a novel microsphere formulation of azithromycin was at least as effective as 7 days of levofloxacin, 500 mg/day, in the treatment of adult patients with mild to moderate community-acquired pneumonia (Fine classes I, II, and III). In total, 427 subjects were randomly assigned to receive either a single 2.0-g dose of azithromycin microspheres (n = 213) or a 7-day regimen of levofloxacin (n = 214). At baseline, 219 of 423 (51.8%) treated subjects had at least one pathogen identified by culture, PCR, or serology. The primary end point was the clinical response (cure or failure) in the "clinical per protocol" population at test of cure (days 13 to 24). Clinical cure rates were 89.7% (156 of 174) for azithromycin microspheres and 93.7% (177 of 189) for levofloxacin (treatment difference, -4.0%; 95% confidence interval, -9.7%, 1.7%). Bacteriologic success at test of cure in the "bacteriologic per protocol" population was 90.7% (97 of 107) for azithromycin microspheres and 92.3% (120 of 130) for levofloxacin (treatment difference, -1.7%; 95% confidence interval, -8.8%, 5.5%). Both treatment regimens were well tolerated; the incidence of treatment-related adverse events was 19.9% and 12.3% for azithromycin and levofloxacin, respectively. A single 2.0-g dose of azithromycin microspheres was at least as effective as a 7-day course of levofloxacin in the treatment of mild to moderate community-acquired pneumonia in adult outpatients.