There has been a pronounced, distinctive increase of the prevalence of obesity within almost all age groups of the Danish population, during the last 25-30 years. The largest increase has been documented in studies based on objective data from total populations and the latest data show the increase will continue. The Danish studies show heterogeneity in the development of the obesity epidemic. A close association with birth cohorts indicates a need for further aetiological research, not only into behavioural factors, but also into early life factors that may explain some of this developmental pattern.