To assess the safety and efficacy of a ten-day oral course of ofloxacin (400 mg 12 hourly) as compared with erythromycin (400 mg every 6 hours) for treatment of lower respiratory tract infections, fifty-two adult outpatients with pulmonary infiltrates (pneumonia) or with a cough and purulent sputum (bronchitis) were evaluated. Expectorated sputum specimens were Gram-stained and cultured, and antibody titres to Mycoplasma pneumoniae, Legionella pneumophilia, and in most cases Chlamydia pneumoniae were measured on acute and convalescent serum samples. Patients were evaluated clinically, microbiologically and radiographically three to five days after concluding therapy; the incidence of adverse reactions was monitored throughout the study period. The ofloxacin group (N = 25) was comprised of nineteen patients with pneumonia and six patients with bronchitis. The erythromycin group (N = 27) was comprised of thirteen patients with pneumonia and fourteen patients with bronchitis. All fifty-two patients were either clinically improved or cured after therapy. Microbiological cure was documented in all fourteen cases (27%) in which causative pathogens were identified. Clinical cure was achieved with ofloxacin in 68% of patients with pneumonia and in 83% of patients with bronchitis, while clinical cure with erythromycin was achieved in 46% of patients with pneumonia and 54% of patients with bronchitis. Adverse reactions (mostly mild gastrointestinal or central nervous system symptoms) were reported by eight patients receiving ofloxacin and four patients receiving erythromycin. While the types of adverse effects were similar, ofloxacin showed a trend toward a higher rate of cure than erythromycin. Ofloxacin is a promising new antibiotic for the treatment of acute lower respiratory infections.