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A recent study showed that during the post-war period the prevalence of extreme overweight among Danish young men was constant until 1960, but since that time it has been continously increasing. We have investigated the underlying changes in the distribution of the body mass index (weight/height2, kg/m2) in this population, consisting of the men obligatorily examined by the draft boards in the metropolitan area from 1943 to 1977 and in an adjacent provincial area from 1964 to 1977. All men are registered before the age of 18, and in the areas and periods in question they made up 400,975, of whom 96% were weighed and measured. A random, 1% sample from the population showed that the central and the lower part of the distribution of the body mass index was unchanged in the entire period of investiga­tion in both areas, the median being approximately 21 and 22 kg/m2, respectively. The upper part of the distribution showed an increased upward skewness beginning around 1960. Inquiry into differences in prevalence of obesity requires knowledge about the underlying distributions in the reference population.