OBJECTIVES: To assess the impact of switching between an ACE Inhibitor (ACEI) and an Angiotensin Receptor Blocker (ARB) on blood pressure (BP) control and cardiovascular  CV events in a real-world setting. METHODS: Records from a longitudinal population-based database of more than 170,000 patients in over 53 family practice clinics in southwestern Ontario, Canada were analyzed.  These records contained chart-abstracted information such as visit diagnosis, BP, medications and consultation notes.  The records from 8,388 adult hypertensive patients treated in mono or combination therapy and who switched between an ACEI and an ARB, (while keeping other agents constant) were included.  Hypertension was defined as a BP exceeding 140/90 mmHg, chart entry of a diagnosis of hypertension, or use of anti-hypertensive medication. Systolic and diastolic BPs and the number of patients who experienced a CV event were recorded. RESULTS:   Reductions in mean systolic BP when patients were switched from ACEI to ARB were 8 mmHg (p<0.002), 7 mmHg (p<0.005), and 8 mmHg (p<0.005) in mono, dual and tri-therapy respectively while patients who were switched from ARB to ACEI had change in of +1 mmHg, -8 mmHg (p<0.001) and -5 mmHg (p<0.005) in mono, dual and tri-therapy respectively. More ARB to ACEI switchers experienced at least one CV event (2.5% versus 0.5% in patients who remain on an ARB monotherapy (p =0.001) and 1.8% versus 0.8% in patients who remain on an ARB based dual therapy (p=0.002)). CONCLUSIONS: In a real-world setting, switching patients from an ACEI to an ARB had a positive impact on mean systolic BP reduction.  When compared to those who remain on an ARB, patients who switched from an ARB to an ACEI experienced more CV events.