The effects of broad-spectrum antibiotic and placebo therapy in patients with chronic obstructive pulmonary disease in exacerbation were compared in a randomized, double-blinded, crossover trial. Exacerbations were defined in terms of increased dyspnea, sputum production, and sputum purulence. Exacerbations were followed at 3-day intervals by home visits, and those that resolved in 21 days were designated treatment successes. Treatment failures included exacerbations in which symptoms did not resolve but no intervention was necessary, and those in which the patient's condition deteriorated so that intervention was necessary. Over 3.5 years in 173 patients, 362 exacerbations were treated, 180 with placebo and 182 with antibiotic. The success rate with placebo was 55% and with antibiotic 68%. The rate of failure with deterioration was 19% with placebo and 10% with antibiotic. There was a significant benefit associated with antibiotic. Peak flow recovered more rapidly with antibiotic treatment than with placebo. Side effects were uncommon and did not differ between antibiotic and placebo.▸ From the Sections of Respiratory and Infectious Diseases, Department of Inte nal Medicine, and the Department of Social and Preventive Medicine, University of Manitoba; Winnipeg, Manitoba, Canada.