Background Hypertension is developed easily in Asian adults with normal body mass index (BMI) (~23 kg/m2), compared with other ethnicities with similar BMI. This study tested the hypothesis that slightly increased BMI at young age is a risk factor for future hypertension in Japanese men by historical cohort study. Methods The study participants were 636 male alumni of the physical education school. They had available data on their physical examination at college age and follow-up investigation between 2007 and 2011. The participants were categorized into six categories: BMI at college age of &lt
20.0 kg/m2, 20.0–21.0kg/m2, 21.0–22.0kg/m2, 22.0–23.0kg/m2, 23.0–24.0kg/ m2, and 24.0kg/m2, and the incidence of hypertension was compared. Results This study covered 27-year follow-up period (interquartile range: IQR: 23–31) which included 17,059 person-years of observation. Subjects were 22 (22–22) years old at graduated college, and 49 (45–53) years old at first follow-up investigation. During the period, 120 men developed hypertension. The prevalence rates of hypertension for lowest to highest BMI categories were 9.4%, 14.6%, 16.1%, 17.5%, 30.3%, and 29.3%, respectively (p&lt
0.001 for trend), and their hazard ratios were 1.00 (reference), 1.80 (95%CI: 0.65–4.94), 2.17 (0.83–5.64), 2.29 (0.89–5.92), 3.60 (1.37–9.47) and 4.72 (1.78–12.48), respectively (p&lt
0.001 for trend). This trend was similar after adjustment for age, year of graduation, smoking, current exercise status and current dietary intake. Conclusion Slightly increased BMI at young age is a risk factor for future hypertension in Japanese men.