Objective The decline in the incidence and mortality of acute myocardial infarction (AMI) has been less among younger compared with older individuals. The aim of this nationwide study was to assess the current incidence, risk factors and outcome of AMI in patients <45 years of age.

Methods All patients ≤80 years of age registered in the Norwegian Myocardial Infarction Register in 2013–2016 were included in this observational, nationwide cohort study. Follow-up was conducted through linkage with the Norwegian Patient Registry through 2017.

Results Among a total of 33 439 patients ≤80 years with AMI, 1468 (4.4%) were <45 years old. The incidence of AMI was 2.1 per 100 000 person-years in people aged 20–29 years, 16.9 in people aged 30–39 years and 97.6 in people aged 40–49 years. Compared with older patients, patients <45 years were more likely to be male (81%), current smokers (56%), obese (30%) and have a family history of premature AMI (44%), and their low-density lipoprotein-cholesterol levels were higher. Patients <45 years were more likely to have non-obstructive coronary artery disease (14% vs 10%, p<0.001) compared with older patients. During a median follow-up time of 2.4 years, 135 (9%) patients <45 years experienced a new AMI, stroke or death, and 58 (4%) patients died.

Conclusions The rate of AMI was low in people <45 years old in Norway, but almost one in ten patients with AMI <45 years old died or experienced a new cardiovascular event during follow-up. Increased efforts to improve risk factor control in these patients are warranted.