ABSTRACT: BACKGROUND: Both obesity and over-expression of HER II are associated with poor prognosis of breast cancer. In vitro experiments suggest that anti-tumour activity of the anti-obesity drug Orlistat is likely to be due to transcriptional suppression of HER II expression. The overexpression of HER II is also positively correlated with other markers of prognosis of breast cancer such as cathepsin expression. HYPOTHESIS: The hypothesis we tested was that the obese women with breast cancer might over-express HER II more often than their lean counterparts to account for the poor prognosis. PATIENTS AND METHODS: One hundred consecutive patients were included in this study. Their body mass indexes were correlated with overexpression of HER II. RESULTS: There was also no association between oestrogen or progesterone receptor positivity and obesity or HER II over expression in premenopausal or post-menopausal women with breast cancer. CONCLUSION: The present study demonstrated that the poor outcome of breast cancer in obese patients is not due to over expression of HER II.