To determine the efficacy and safety of pefloxacin in the treatment of lower respiratory tract infections, a multicentre trial involving four departments of respiratory diseases was performed. One hundred and eight patients were admitted to the study: most of them were affected with exacerbations of chronic bronchitis or with pneumonia complicating lung cancer. Isolation and identification of responsible microorganisms from bronchial secretions was possible in 78 patients. Seven patients were withdrawn, one for worsening of the underlying disease and six for early side-effects. Thus, of 108 patients recruited, 101 completed the course of therapy (pefloxacin 400 mg bd for 5-14 days) and could be submitted to final evaluation. Of these 43 (42.6%) were cured and 48 (47.5%) showed improvement. Eradication of responsible microorganisms was achieved in 70 (90.9%) of 77 patients microbiologically evaluated. Side-effects of moderate severity were observed in 12 patients (gastrointestinal disturbances in 11 and dyspnoea in one); these did not necessitate discontinuation of therapy.