The obesity epidemic, evolving in many countries since the 1970s, has been attributed to the widespread contemporary so-called obesogenic transformation of the societies, but what preceded the epidemic? Using quantile regression, we studied the trends by year of birth in the percentile distribution of body mass index (BMI = weight/height 2) of 320,962 Danish school children, born from 1930 to 1976, and of 205,153 Danish young conscripts, born from 1939 to 1959. The overall trend of the percentiles of the BMI distributions were found to be linear across the years of birth. While the percentiles below the 75th were almost stable, those above showed a steadily steeper rise the more extreme the percentile among both school children and young men is. These changes, indicating the emergence of the obesity epidemic, preceded the presumed obesogenic transformation of the society by several decades and imply that other, so far unknown, factors have been involved.