OBJECTIVES: To estimate the avoidable health care costs of uncontrolled hypertension in Canada. METHODS: We used an established cost of illness model to estimate cost of cardiovascular (CV) hospitalisations at current prevalence of hypertension to the Canadian health care system, and the level of these costs if hypertension was successfully treated to target levels. Two populations were examined: hypertensives in the general population; and a subpopulation with both hypertension and Non-Insulin Dependent Diabetes Mellitus (NIDDM). Population data were sourced from the 2001 census. Prevalence of hypertension in the general population was taken from the Canadian Heart Health Survey and in NIDDM was calculated from Diabetes in Canada and Canadian Diabetes Association data. The HOT trial and Framingham Risk Profile equation were used to examine the relationship between achieved blood pressure and 3 major CV events - stroke, congestive heart failure (CHF), and symptomatic acute myocardial infarction (AMI). Cost of CV events were taken from the Ontario Case Costing Project (AMI and CHF) and the Canadian Economic Burden of Illness Study (stroke). Stochastic simulation was used to construct confidence intervals. RESULTS: The model estimated that 5.3 million Canadians over age 18 have hypertension. Uncontrolled hypertension was associated with 17,000 excess CV events and costs of C$306 million (95%CI: C$154-632m) annually. Among 309,000 hypertensives with NIDDM, uncontrolled hypertension was associated with 4,000 excess CV events annually and costs of C$68 million (95%CI: C$37-103m). CONCLUSIONS: Uncontrolled hypertension contributes substantially to the overall cost of cardiovascular disease in Canada. Diabetic hypertensives represented 22% of the avoidable cost of hypertension and 6% of the hypertensives population.