In the international Drug Education Project, an educational program involving auditing and feedback in peer groups to improve the treatment of asthma and urinary tract infections (UTI) was developed and tested in primary care. Individualized feedback was provided and discussed in 24 Dutch peer groups showing doctors their prescribing practices and underlying reasons for treatment. A parallel, randomized controlled design was used to test the effect on competence and actual prescribing; in one study arm doctors received feedback on asthma treatment and in the other on UTI treatment. Especially the messages to treat asthma exacerbations with oral corticosteroids (17% increase) and to prescribe short courses for UTI (decrease duration of 1.8 days) brought about large improvements. Both messages concerned acute situations, and were clear and relatively easy for GPs to implement. GPs will experience more barriers when changing maintenance treatment of an asthma patient, which could explain the more limited success of this part of the educational program: the proportion of patients treated with inhaled corticosteroids increased 5%. A ceiling effect was experienced regarding drug choice for UTI.