Control charts are tools from the field of statistical process control for visualizing the longitudinal development of quality indicators, and detecting whether the underlying process is changing. They have been used in critical care and disease management settings to monitor and improve patient outcomes. This paper investigates the application of control charts to monitor adherence to clinical practice guidelines by healthcare professionals. Data were used from a recent trial on computerized decision support in outpatient cardiac aftercare. Guideline adherence increased in clinics that started using decision support. A gradual drop in adherence was seen in clinics that continued using decision support over a longer period. Control charts are more sensitive to detect changes in adherence than summary comparisons in before-after designs.