BACKGROUND: Understanding myocardial infarction (MI) incidence and case fatality trends across the four UK constituent countries is of importance following devolution of the government of health-care services. METHODS: Retrospective cohort study using a primary care database (5.19 million patients) examining trends in incidence of first MI and 30-day case fatality. RESULTS: From 1996 to 2005, the incidence of MI decreased in all countries, but reductions were greater in England (men, -3.1%; women, -2.8%) and Wales (men, -3.3%; women, -4.6%) than in Scotland (men, -1.9%; women, -0.6%) and Northern Ireland (men no change, women, -0.8%) (average annual percentage change). Greater reductions in England and Wales than Scotland and Northern Ireland meant a widening of north-south difference in MI incidence over the study period. Downward trends in 30-day case fatality were found in each country but less regional variation was evident (England men, -12.0%, women, -11.0%; Wales men, -18.4%, women, -12.6%; Scotland men, -9.5%, women, -9.0%; Northern Ireland men, -8.6%, women, -13.0%). CONCLUSION: From 1996 to 2005, downward trends in the incidence of first MI and 30-day case fatality were evident in each constituent country. Greater improvements in case fatality, compared with incidence, were found within each country.