Courses of 7 days on either 200 mg pivmecillinam/250 mg pivampicillin given twice daily or 250 mg amoxycillin given three times daily were compared in 3783 general practice patients with acute exacerbations of chronic bronchitis in a single-blind parallel group study. Patients on pivmecillinam/pivampicillin had significantly more 'excellent' responses, although there was no difference in overall response between treatments. Post-treatment sputum colour and consistency and peak expiratory flow rates were significantly better with pivmecillinam/pivampicillin. The incidence of side-effects was the same for both groups (approximately 10%), although significantly more patients reported lower gastro-intestinal problems with amoxycillin. Treatment was withdrawn due to side-effects in 47 (2.5%) patients on pivmecillinam/pivampicillin and 51 (2.7%) patients on amoxycillin. Amoxycillin is a standard therapy in the treatment of patients with acute exacerbations of chronic bronchitis. In this study, however, it was shown that pivmecillinam/pivampicillin offers benefits over amoxycillin in these patients.