Patients with Gram-negative lower respiratory tract infections (acute exacerbation of chronic bronchitis (n = 23), pneumonia (n = 4), and bronchiectasis (n = 5) were treated with pefloxacin, 400 mg twice daily, given either intravenously or orally. Symptoms, signs and sputum volume and colour were monitored daily. Chest X-rays, sputum culture and Gram-stain examinations were carried out on days 1 and 5, and immediately after the end of the treatment. There was a clinical improvement, as indicated by the incidence of cough, dyspnoea and rales, and by sputum volume and colour in 31 patients (97%). Microbiological improvement, as indicated by the complete elimination of sputum pathogens and pus cells, was achieved in 28 of the patients (88%). In one patient, an adverse effect, renal failure, occurred. These results suggest that pefloxacin is both clinically and microbiologically effective for the treatment of Gram-negative lower respiratory tract infections.