The subjects were 6,033 outpatients, from every province of Spain, with acute bronchitis (44%), exacerbation of chronic bronchitis (35%), typical pneumonia (14%), or atypical pneumonia (8%). Most of the patients were aged 50 to 70 years. The dose of josamycin in over 90% of the patients was 2 gm daily. The mean duration of infection before treatment was 5.3 days and treatment lasted a mean of 9.2 days. Concomitant drugs were taken by 78% of the patients; these included mucolytic agents, xanthine derivatives, beta-adrenergic agonists, and corticosteroids. After two weeks of treatment, the infection was cured in 82% of the patients with acute bronchitis, in 30% with chronic bronchitis, in 85% with typical pneumonia, and in 85% with atypical pneumonia; improvement was shown by 16%, 66%, 13%, and 13%, respectively. Treatment side effects were noted in 11% of the patients receiving josamycin alone and in 17% of the patients receiving concomitant drugs. Most side effects were mild and transient; treatment was discontinued because of side effects in 2% of the patients receiving josamycin alone and in 3% receiving concomitant drugs. It is concluded that josamycin is a safe and effective agent in the treatment of bronchopulmonary infections.