Abstract Background Increasing numbers of stroke cases in the young are reported in many countries, potentially at least partly due to rising rates of overweight and obesity. Most data on the effect of obesity on stroke is derived from middle aged populations; there is a comparative lack of information on the effect of excessive body weight in young people. Purpose We aimed to determine whether body mass index (BMI) at age 18 predicted early stroke events among men in Sweden. Methods Population-based Swedish cohort study of conscripts (n=1,668,921; mean age at baseline, 18.3 years) who enlisted during 1968–2005. Follow-up was done through linkage to the nationwide Swedish patient and death registries. Risk of a first hospitalization for stroke (any type) during follow-up (5–46 years) was calculated with Cox proportional hazards models. Objective baseline measures of fitness and cognition were included in the models in a second set of analyses. Results During follow-up there were 12,512 first hospitalizations for stroke (mean age at diagnosis, 48.5 (SD 9.4) years, maximum 64 years). Compared with men with BMI of 18.5 to 20.0 kg/m2, men with BMI 20 to <22.5 and 22.5 to <25.0 kg/m2 had hazard ratios (HR) of 1.06 (1.00–1.12) and 1.23 (1.14–1.31), respectively, for hospitalization for stroke, after adjustment for age, year of conscription, comorbidities at baseline, parental education, blood pressure, IQ, muscle strength, and fitness. Those with a BMI of ≥35 kg/m2 had an HR of 3.33 (2.46–4.51) for an event before the age of 65, compared to slim men. The multiple-adjusted risk per 1-unit increase in BMI was 1.07 (95% CI, 1.06–1.08). Conclusion We found a rise in risk of early hospitalisation for stroke detectable already at normal levels of body weight at age 18, and rising to more than 3-fold in the highest weight category. Given increasing levels of body weight, and prevalence of overweight, obesity, and severe obesity in young adults, the increased incidence of stroke in the young may potentially partly be explained by rising body weight, with an obvious potential for prevention.