OBJECTIVE To compare the incidence and case fatality of acute myocardial infarction in Denmark and Sweden. METHODS A cohort study, linking the national registries of hospital admissions and causes of death in the two countries. METHODS All admissions and deaths with acute myocardial infarction as primary or secondary diagnosis were extracted (Denmark, 1978 to 1998; Sweden, 1987 to 1999). METHODS The incidence was estimated using the first acute myocardial infarct for each patient. Case fatality was estimated in the first 28 days after acute myocardial infarction, including prehospital deaths. All rates were adjusted for age. RESULTS The incidence of myocardial infarction and the case fatality declined significantly among all subgroups of patients. Case fatality was higher in Denmark early in the study period (1987-1990) than in Sweden. The odds ratios (OR) ranged from 1.28 to 1.50 in the four age groups. In 1994-1999, the prognosis of patients younger than 75 years did not differ. Patients aged 75-94 years still fared worse in Denmark (OR 1.21, 95% confidence interval 1.17 to 1.27). Women aged 30-54 years had a worse prognosis than men in both Denmark and Sweden (OR associated with male sex 0.85 and 0.90, respectively). In contrast, for patients older than 65 years, women had a better prognosis than men. This difference in the effect of sex with age was significant (p < 0.0001) and did not change over time. CONCLUSIONS Case fatality after acute myocardial infarction was notably higher in Denmark than in Sweden in 1987-1991, but in the later periods the prognosis was comparable in the two countries.