Objective: To examine how physicians in Ontario, Canada, have altered their referral patterns for coronary angiography after acute myocardial infarction (AMI) over time. Design: Retrospective analysis of multilinked administrative data. Setting: Province of Ontario, Canada. Patients: 146 365 Ontario AMI patients hospitalised between 1 April 1992 and 31 March 1999. Main outcome measures: Utilisation trends of coronary angiography among all patients, as well as within six subgroups: elderly (versus young), women (versus men), high (versus low) risk of 30 day mortality, high (versus low) socioeconomic status, cardiology (versus non-cardiology) attending physician specialty, and hospitals with (versus without) onsite revascularisation capacity. Cox proportional hazard models were adjusted for variations in patient, physician, and hospital characteristics over time. Results: Angiography rates in Ontario increased from 23.2% in 1992 to 35.5% in 1999 (p < 0.0001). Increases in utilisation of coronary angiography were most pronounced among the elderly (12.4–24.3% v 39.3–54.4% for non-elderly patients, p < 0.0001), the affluent (24.6–38.7% v 22.0–32.3% for less affluent patients, p = 0.01), and those tended to by cardiologists (32.0–47.1% v 20.3–30.1% for non-cardiology attending specialties, p < 0.0001) after adjusting for changes in baseline patient, physician, and hospital characteristics over time. Conclusions: Despite universal health care availability, not all patients benefited equally from increases in service capacity for coronary angiography after AMI in Ontario. Wider implementation of data monitoring and explicit management systems may be required to ensure that appropriate utilisation of cardiac services is allocated to patients who are most in need.