BACKGROUND: The role of antibiotics in treatment of patients with moderate exacerbations of chronic obstructive pulmonary disease (COPD) is uncertain, but such treatment might be useful in very severe episodes. Our objective was to assess the effects of ofloxacin in patients with exacerbations of COPD who required mechanical ventilation. METHODS: We did a prospective, randomised, double-blind, placebo-controlled trial in 93 patients with acute exacerbation of COPD who required mechanical ventilation. Patients were randomly assigned to receive oral ofloxacin 400 mg once daily (n=47) or placebo (46) for 10 days. Primary endpoints were death in hospital and need for an additional course of antibiotics, both separately and in combination. Analysis was by intention to treat. FINDINGS: Three patients dropped out of the study. Two (4%) patients receiving ofloxacin died in hospital and ten (22%) did so in the placebo group (absolute risk reduction 17.5%, 95% CI 4.3-30.7, p=0.01). Treatment with ofloxacin significantly reduced the need for additional courses of antibiotics (28.4%, 12.9-43.9, p=0.0006). The combined frequency of death in hospital and need for additional antibiotics was significantly lower in patients assigned to ofloxacin than in those receiving placebo (45.9%, 29.1-62.7, p<0.0001). The duration of mechanical ventilation and hospital stay was significantly shorter in the ofloxacin group than in the placebo group (absolute difference 4.2 days, 95% CI 2.5-5.9; and 9.6 days, 3.4-12.8, respectively). INTERPRETATION: New fluoroquinolones, such as ofloxacin, are beneficial in the treatment of COPD exacerbation requiring mechanical ventilation.