Objectives To examine the association between body mass index (BMI) in young adulthood and cardiovascular risks, including venous thromboembolism, before 55 years of age.

Design Cohort study using population-based medical databases.

Setting Outcomes registered from all hospitals in Denmark from 1977 onwards.

Participants 6502 men born in 1955 and eligible for conscription in Northern Denmark.

Main outcome measures Follow-up began at participants’ 22nd birthday and continued until death, emigration or 55 years of age, whichever came first. Using regression analyses, we calculated the risks and HRs, adjusting for cognitive test score and years of education.

Results 48% of all obese young men (BMI ≥30 kg/m²) were either diagnosed with type 2 diabetes, hypertension, myocardial infarction, stroke or venous thromboembolism or died before reaching 55 years of age. Comparing obese men with normal weight men (BMI 18.5 to <25.0 kg/m²), the risk difference for any outcome was 28% (95% CI 19% to 38%) and the HR was 3.0 (95% CI 2.3 to 4.0). Compared with normal weight, obesity was associated with an event rate that was increased more than eightfold for type 2 diabetes, fourfold for venous thromboembolism and twofold for hypertension, myocardial infarction and death.

Conclusions In this cohort of young men, obesity was strongly associated with adverse cardiometabolic events before 55 years of age, including venous thromboembolism. Compared with those of normal weight, young obese men had an absolute risk increase for type 2 diabetes, cardiovascular morbidity or premature death of almost 30%.