Aims To compare short- and long-term mortality after a first acute myocardial infarction (AMI) in patients with and without diabetes mellitus. Methods and results A nationwide cohort of 2,018 diabetic and 19,547 nondiabetic patients with a first hospitalized AMI in 1995 was identified through linkage of the national hospital discharge register and the population register. Follow-up for mortality lasted until the end of 2000. At 28 days and 5 years respectively, absolute mortality risks were 18 and 53% in diabetic men, 12 and 31% in nondiabetic men, 22 and 58% in diabetic women, and 19 and 42% in nondiabetic women. Crude mortality was significantly higher in diabetic patients than in nondiabetic patients in both men (28-day hazard ratio (HR) 1.55; 95% confidence interval (CI) 1.32-1.81, 5-year HR 2.01; 95% CI 1.84-2.21) and women (28-day HR 1.19; 95% CI 1.03-1.37, 5-year HR 1.53; 95% CI 1.40-1.67). After multivariate adjustment, risk differences became nonsignificant at 28 days, but diabetes was still associated with a significantly higher long-term mortality in both men (28-day HR 1.16; 95% CI 0.99-1.36, 5-year HR 1.49; 95% CI 1.36-1.64) and women (28-day HR 1.12; 95% CI 0.97-1.28, 5-year HR 1.39; 95% CI 1.27-1.52). The interaction between diabetes mellitus and gender did not reach significance in the analyses. Conclusion Our findings in an unselected cohort covering a complete nation show a significantly higher long-term mortality after a first acute myocardial infarction in diabetic patients. Yet, short-term mortality is not significantly higher in diabetic patients. Risks appear to be equally elevated in men and women.