OBJECTIVE: To assess whether the composition of the obese category (body mass index (BMI)> or =30) has changed during the last one-third of a century in young adult men. DESIGN: Retrospective study of 1,580,913 men (18.3+/-0.4 years) representing 82% of the Swedish male population at military conscription age between 1969 and 2005. Measured height and weight were used to define moderate and morbid obesity as BMI 30-34.9 and > or =35, respectively. Data on socio-economic position (SEP), place of residence (urban, semi-urban and rural), age and test center were also collected. RESULTS: From the period 1969-1974 to 2000-2005, the prevalence of moderate obesity almost quintupled (0.8-3.8%; P