While most studies show a higher body mass in Western women to be positively associated with an increased breast cancer risk in postmenopausal women, they show a negative association in the case of premenopausal women. A review of case-control and cohort studies suggest that such protection applies mainly to obesity in teenage girls, whereas obesity appearing after the teenage years is more likely to be associated with a higher risk of postmenopausal breast cancer. The mechanisms are uncertain. There is evidence that obesity and the components of the Western diet can independently provoke hyperinsulinaemic insulin resistance at puberty, and in adolescent girls this has been related to evidence of abnormal ovarian steroidogenesis and anovulation. This may decrease promotion of mammary carcinogenesis. If however, obesity continues after the teenage years, the higher concentration of insulin-like growth factor 1 (IGF1) associated with hyperinsulinaemia can interact with oestrogen receptors in mammary epithelium to lead to increased proliferative activity. This review postulates that the observed protective effect of early obesity against premenopausal breast cancer is likely to be replaced by an increased risk of postmenopausal breast cancer if obesity continues after the teenage years. The manifestation of breast cancer is merely postponed to an older age. Recent prospective and case-control studies suggest that increased bioavailability of IGF1 is a marker of increased breast cancer risk in premenopausal women. Nutritional intake in early life may programme later activity in the growth hormone-IGF1 axis and influence the progression of transformed cells in mammary tissue. The question remains whether deliberate weight loss can reverse the effects of weight gain.