Aims Family history is an established risk factor for myocardial infarction (MI), but it is not clear how this risk changes with number and gender of first-degree relatives with MI. We used the entire Danish population to examine the importance of MI in siblings and parents.

Methods and results This study is a retrospective nationwide register-based cohort study including registered relatives to all Danish citizens diagnosed with MI in the period 1978–2010. In the entire Danish population we identified siblings to 7552 patients with a first-time MI. The rate ratios (RR) calculated by Poisson models showed an RR of 4.30 (95% confidence interval 3.53–5.23) for siblings of a patient with MI. Children of parents with MI also showed high risk: for children of a maternal case RR 2.40 (2.20–2.60), and of a paternal case RR 1.98 (1.98–2.09), respectively; P value for gender interaction <0.0001. A paternal case with MI at an age <50 years was associated with an RR of 3.30 (2.92–3.72) while a case >50 years was associated with a risk of 1.83 (1.73–1.93). For maternal cases below and above 50 years of age the risks were 3.23 (2.56–4.10) and 2.31 (2.11–2.52), respectively.

Conclusion First-degree relatives of a patient with myocardial infarction themselves have a substantial higher risk of myocardial infarction. The risk is particularly elevated when the MI case is the mother or a sibling, and when the MI case has the infarction before the age of 50 years.