A random sample of patients aged twenty to fifty-nine on the lists of two general practitioners in the London area was weighed and measured. It was found that thirty-seven per cent of the men and forty-nine per cent of the women were at least twenty per cent above the desirable weight for their height, and eighteen per cent of the men and twenty-five per cent of the women were at least thirty per cent above this desirable weight. The prevalence of obesity was related to age and to social class, particulary among the women. Seventy-two per cent of women in the lower socio-economic group were overweight compared to thirty-nine per cent of upper class women. There was also a greater prevalence of obesity among the older subjects in the sample. These findings are discussed in relation to the pathogenesis of obesity.