Over the past few decades the proportion of people with excess body weight has been increasing in both developed and less developed countries. About 50% of men and 35% of women in Europe are currently estimated to be overweight or obese. In addition to an increase in the risk of cardiovascular disease and type II diabetes, the evidence summarised here shows that excess body weight is directly associated with risk of cancer at several organ sites, including colon, breast (in postmenopausal women), endometrium, oesophagus, and kidney. In part, these associations with cancer risk may be explained by alterations in the metabolism of endogenous hormones-including sex steroids, insulin, and insulin-like growth factors-which can lead to distortion of the normal balance between cell proliferation, differentiation, and apoptosis. Avoidance of weight gain thus seems to be an important factor for cancer prevention.