Thirty four patients with acute purulent exacerbations of chronic bronchitis were treated with 500 mg ciprofloxacin twice daily, orally, for ten days. Short time cure rate was 97% (cure 71%, improvement 26%) and failure 3%, long time cure rate (six months follow-up) was 74%. Predominant initial pathogens wereHaemophilus influenzae andStreptococcus pneumoniae, mostly in pure cultures. All sputum cultures except those withStreptococcus pneumoniae became negative on the third day of treatment. Apart from a slower clearance of pneumococci from sputum there were no significant differences in responses between pneumococcal andHaemophilus infections during and after therapy. Mild adverse gastrointestinal effects were noticed in five patients.