Most published studies suggest that the rising prevalence of overweight and obesity is a recent phenomenon. We have investigated whether secular trends in body mass index (BMI) existed in the 1950s and 1960s, using data collected from young adults attending Glasgow University. Mean BMI increased from 1948 to 1968 in men, and decreased slightly in women. The proportion of men who were overweight increased, whereas that of women changed little. These data must be interpreted in light of the fact that the study participants were relatively affluent, and not representative of the whole population in terms of socioeconomic position in childhood. The increases of BMI in men evident from 1948 to 1968 suggest that recent changes in exercise and dietary patterns do not fully explain changes in body weight over time.