100 hospital patients suffered from acute exacerbations of chronic bronchitis. 50 were treated with amoxycillin in a dose of 500 mg, three times a day for 10 days and the results compared with 50 patients treated with co-trimoxazole in a dose up to 480 mg trimethoprim and 2,400 mg of sulphamethoxazole daily in males, and two thirds of this dose in females. The trial was single-blind. During the acute phase of infection, both treatments were equally effective in clinical improvement, conversion of the sputum from purulent to mucoid, diminution of quantity and elimination of pathogenic bacteria. Amoxycillin was quicker in sputum conversion and gave less side effects, but the differences were not significant. During the 2-4 weeks following treatment, only a third of the patients who had received co-trimoxazole remained well and free from purulent relapse, as opposed to 72% who had received amoxycillin, a difference significant at the 2% level.