Body mass index (weight/height 2; kg/m2) of 38,132 Danish young men, representing approximately 10 percent of those undergoing mandatory draft board examination was analyzed with regard to secular changes through the birth cohorts 1939-58 in the Greater Copenhagen area and a western provincial region, and through the cohorts 1947-58 in two other provincial areas. The 25th, 50th, and 75th percentiles of body mass index remained essentially unchanged over time. The prevalence of obesity (body mass index greater than or equal to 30 kg/m2) showed a steep increase in birth cohorts beginning in the early 1940s and a levelling off thereafter. The secular increase in prevalence of obesity without similar trends in median body mass index strongly suggest the existence of environmental causes of obesity to which the study population was increasingly exposed. The changes may have occurred either in specific causes of obesity or, most likely, in more general environmental conditions causing obesity only together with genetic predisposition, for which there is growing evidence.