Ciprofloxacin and amoxycillin were compared in the treatment of acute exacerbations of chronic obstructive airways disease. One hundred and forty patients were evaluable for overall outcome: 60 patients' sputum specimens yielded positive bacteriological cultures. The predominant pathogens were Haemophilus influenzae (43.3%) and Streptococcus pneumoniae (25.0%). Both regimens were safe and produced few adverse effects. An independent observer judged ciprofloxacin to produce a 91.8% success rate (complete success 21.9%; partial success 69.9%) whilst a 73.1% rate (complete success 10.4%; partial success 62.7%) was seen with amoxycillin.