This paper analyses the causal effect of education on the probability of being overweight by using longitudinal data of Australian identical twins. The data include self-reported and clinical measures of body size. The prevalence of overweight and obesity is growing rapidly in many countries. Education policies might be important for reducing this increase. Our cross-sectional estimates confirm the well-known negative association between education and the probability of being overweight. For men we find that education also reduces the probability of being overweight within pairs of identical twins. The estimated effect of education on overweight status increases with age. Remarkably, for women we find no negative effect of education on body size when fixed family effects are taken into account. Identical twin sisters that differ in educational attainment do not systematically differ in body size. This finding is robust to differences in employment and number of children.