An international multi-centre, randomized, prospective, double-blind study compared oral moxifloxacin (200 mg or 400 mg once daily for 10 days) with oral clarithromycin (500 mg, twice daily for 10 days) in the treatment of community-acquired pneumonia (CAP). The clinical success rate in the evaluable population at the primary efficacy assessment, 3-5 days after the end of study treatment, was 93.9% in patients treated with 200 mg moxifloxacin; 94.4%, with 400 mg moxifloxacin; and 94.3%, with clarithromycin. Clinical success rates were maintained at follow-up, 21-28 days after the end of treatment: 90.7% (200 mg moxifloxacin), 92.8% (400 mg moxifloxacin) and 92.2% (clarithromycin). The 95% confidence intervals indicated that all three treatment regimens were equally effective in treating CAP. At follow-up, the 400 mg moxifloxacin dose had a slightly higher observed cure rate than the 200 mg moxifloxacin dose, but this was not statistically significant. The most frequently isolated pathogens were Streptococcus pneumoniae (42%), Haemophilus influenzae (19%), Haemophilus parainfluenzae (10%), Moraxella catarrhalis (6%), Klebsiella pneumoniae (5%) and Staphylococcus aureus (4%). The bacteriological success rate (eradication and presumed eradication) was 72.5% (29/40) for 200 mg moxifloxacin, 78.7% (37/47) for 400 mg moxifloxacin and 70.7% (29/41) for clarithromycin. The adverse event profile was comparable between the three treatment groups. Most adverse events, possibly or probably related to the study drug, were generally mild or moderate in severity and mostly related to the digestive system: diarrhoea, nausea and abdominal pain in 200 mg moxifloxacin patients; diarrhoea, liver function abnormalities and nausea in 400 mg moxifloxacin patients and liver function abnormalities, diarrhoea, nausea and taste perversion in clarithromycin patients. Study drugs were discontinued because of adverse events in 7/229 (3%) patients treated with 200 mg moxifloxacin, 11/224 (5%) with moxifloxacin 400 mg and 11/222 (5%) with clarithromycin. In all assessments, moxifloxacin was at least as effective clinically, and as well tolerated as clarithromycin in the treatment of CAP. Bacteriological success rates in moxifloxacin-treated patients were greater than those of clarithromycin. Moxifloxacin, given once daily, is free of many drug-drug interactions and requires no dosage adjustments in most renal hepatic deficient patients.