Prophylactic antibiotics in chronic bronchitis / COPD have a small but statistically significant effect in reducing the days of illness due to exacerbations of chronic bronchitis. They do not have a place in routine treatment because of concerns about the development of antibiotic resistance and the possibility of adverse effects. The available data are over 30 years old, so the pattern of antibiotic sensitivity may have changed and there is a wider range of antibiotics in use.