We conducted a double-blind, randomized, placebo-controlled trial in 40 patients to evaluate the need for antibiotics in acute exacerbations of chronic bronchitis. All patients were sufficiently ill to require hospitalization although none needed ventilatory support; the presence of pneumonia was excluded. Treatment consisted of bronchodilators, corticosteroids, and either tetracycline, 500 mg, or placebo by mouth every 6 hours for 1 week. Arterial blood gases, spirometric tests, bacteriologic evaluation of sputum, and patient and physician evaluation of the severity of illness were assessed at the beginning and end of the study. All patients improved both symptomatically and by objective measures of lung function. At the end of the study period there were no differences between those patients receiving tetracycline and those receiving placebo. We conclude that antibiotic therapy is not needed in moderately ill patients with exacerbations of chronic bronchitis.