BACKGROUND: Variations in the prevalence of traditional cardiac risk factors only partially account for geographic variations in the incidence of cardiovascular disease. We examined the extent to which preventive ambulatory health care services contribute to geographic variations in cardiovascular event rates. METHODS:We conducted a cohort study involving 5.5 million patients aged 40 to 79 years in Ontario, Canada, with no hospital stays for cardiovascular disease as of January 2008, through linkage of multiple population-based health databases. The primary outcome was the occurrence of a major cardiovascular event (myocardial infarction, stroke or cardiovascularrelated death) over the following 5 years. We compared patient demographics, cardiac risk factors and ambulatory health care services across the province's 14 health service regions, known as Local Health Integration Networks (LHINs), and evaluated the contribution of these variables to regional variations in cardiovascular event rates.RESULTS: Cardiovascular event rates across LHINs varied from 3.2 to 5.7 events per 1000 person-years. Compared with residents of high-rate LHINs, those of lowrate health regions received physician services more often (e.g., 4.2 v. 3.5 mean annual family physician visits, p value for LHIN-level trend = 0.01) and were screened for risk factors more often. Lowrate LHINs were also more likely to achieve treatment targets for hypercholesterolemia (51.8% v. 49.6% of patients, p = 0.03) and controlled hypertension (67.4% v. 53.3%, p = 0.04). Differences in patient and health system factors accounted for 74.5% of the variation in events between LHINs, of which 15.5% was attributable to health system factors alone. INTERPRETATION:Preventive ambulatory health care services were provided more frequently in health regions with lower cardiovascular event rates. Health system interventions to improve equitable access to preventive care might improve cardiovascular outcomes.