Cardiac rehabilitation is a multidisciplinary therapy aimed at recovery and secondary prevention after hospitalization for cardiac incidents (such as myocardial infarctions) and cardiac interventions (such as heart surgery). To stimulate implementation of the national guidelines, an electronic patient record system with computerised decision support functionalities called CARDSS (cardiac rehabilitation decision support system) was developed, and made available to Dutch rehabilitation clinics. The system was quantitatively evaluated in a cluster randomised trial at 31 clinics, and qualitatively by interviewing 29 users of the system. Computerised decision support was found to improve guideline concordance by increasing professional knowledge of preferred practice, by reducing inertia to previous practice, and by reducing guideline complexity. It was not effective when organizational or procedural changes were required that users considered to be beyond their responsibilities.