Objective To determine the extent to which antibiotics reduce the risk of serious complications after common respiratory tract infections. Design Retrospective cohort study. Setting UK primary care practices contributing to the general practice research database. Data source 3.36 million episodes of respiratory tract infection. Main outcome measures Risk of serious complications in treated and untreated patients in the month after diagnosis: mastoiditis after otitis media, quinsy after sore throat, and pneumonia after upper respiratory tract infection and chest infection. Number of patients needed to treat to prevent one complication. Results Serious complications were rare after upper respiratory tract infections, sore throat, and otitis media, and the number needed to treat was over 4000. The risk of pneumonia after chest infection was high, particularly in elderly people, and was substantially reduced by antibiotic use, with a number needed to treat of 39 for those aged =65 and 96-119 in younger age groups. Conclusion Antibiotics are not justified to reduce the risk of serious complications for upper respiratory tract infection, sore throat, or otitis media. Antibiotics substantially reduce the risk of pneumonia after chest infection, particularly in elderly people in whom the risk is highest.