In a non-concurrent cohort study, the data from medical examinations of 78,612 Dutch men at the age of 18 were linked to mortality registrations over a follow-up period of 32 years. Body weight and height, blood pressure, resting pulse rate, education, birth order and a health score were recorded. The average Body Mass Index was 20.8 kg/m2. About 5% of the men were very lean (BMI less than 18) and only about 2% had grade I obesity (BMI greater than 25). During 32 years of follow-up 3642 men died. By means of a logistic regression analysis the impact of Body Mass Index (BMI) at the age of 18 on longevity was investigated. Healthy, well-educated men with a BMI of 19 had a higher rate of survival during the entire follow-up period. A negative effect of a high BMI (greater than 25) was only demonstrable after 20 years of follow-up. Men with a BMI under 18 had an increased mortality risk, which was mainly caused by an impaired health status. The results of this study support the hypothesis that moderately obese young men have an increased mortality risk.