Environmental factors promote the development of and decrease survival from Breast Cancer. Prospective morphological and hormonal studies indicate biological markers for this disease are evident in premenopausal women. The majority of premenopausal patients are non-obese (Body Mass Index, BMI less than 25). Lean women have a greater proportion of estrogen receptor negative (ER-) tumours, which may grow faster and have a higher concentration of epithelial growth factor (EGF). We have reported that lean, BMI less than 23, versus obese, BMI greater than 28, women have a different gut-pancreatic peptide hormone response to meals and that differences in these peptide hormones occur between healthy and age weight matched premenopausal patients. We hypothesize that the diet peptide hormone control of food intake in lean women is associated with the development of mammary dysplasia, change in growth factor profile and steroid hormone metabolism.