Background: Weight gain is commonly reported by patients with breast cancer. Aromatase inhibitors are being tested in the preventive setting in high risk women and it is important to evaluate the association between treatment and weight change as this may effect the patient’s decision to continue to take the drug. Methods: Weight change in postmenopausal women from three large clinical trials (ATAC, IBIS-I, IBIS-II) investigating endocrine treatment for the treatment and prevention of breast cancer have been analysed. The objective of this retrospective study was to assess the effects of anastrozole and tamoxifen on weight change in postmenopausal women. Results: In the ATAC trial, a mean increase of 1.4 kg was observed after 12 months of follow-up and no statistically significant differences between treatment arms (anastrozole vs. tamoxifen) were found. The majority of women kept their weight stable and only 11.2% gained more than 5 kg within this time period. Significant baseline predictors for gaining more than 5 kg of weight after 12 months of follow-up were being younger than 60 years old at entry, smoking at entry, and mastectomy. Results for the two prevention studies IBIS-I and IBIS-II were quite similar. In the IBIS-II trial, the overall mean weight change after 12 months of follow-up was 0.5 kg. In the IBIS-I study women gained an average of 0.9 kg within the first 12 months of follow up. In both studies no differences between treatment and placebo (IBIS-I: tamoxifen vs. placebo; IBIS-II: anastrozole vs. placebo) were observed. Overall, women in the adjuvant setting gained more than 5 kg of weight (11.2%) after 12 months of follow up compared to those in the preventive setting (5.3% IBIS-I, 5.7% IBIS-II), but similar overall weight gain was seen after 60 months of follow up in both settings (5.7% IBIS-I vs. 6.1% ATAC). Conclusions: All three trials have demonstrated that weight gain occurs primarily within the first 12 months of active treatment and are similar to those on placebo. Weight stabilises after 12 months of follow up and major weight gain was rare in all trials, and unrelated to treatment.