The effectiveness of ciprofloxacin was compared with that of cotrimoxazole in the treatment of acute bacterial exacerbations of chronic obstructive pulmonary disease. 65 patients were eligible for efficacy analysis and the cure rate (comprised of both complete and partial success responses) was 97.4% for ciprofloxacin-treated patients and 92.6% for patients treated with cotrimoxazole. 261 causative organisms were isolated from 216 patients valid for safety analysis, of which 4 (from 4 patients) were resistant to ciprofloxacin and 70 (from 55 patients) were resistant to cotrimoxazole.Adverse events occurred with an almost identical frequency in the ciprofloxacin- and cotrimoxazole-treated groups (22.3 and 23.9%, respectively). Ciprofloxacin is a well tolerated and effective antibiotic in patients with acute bacterial exacerbations of chronic obstructive lung disease. Causative bacterial pathogens are rarely resistant to ciprofloxacin but commonly resistant to cotrimoxazole.