Background . A breast cancer diagnosis has been suggested as a teachable moment when a woman is more open to making healthier lifestyle changes. Little is known about the health behaviour changes women with breast cancer initiate compared to those made by other women. Material and methods . We examined changes in body mass index (BMI) and tobacco and alcohol consumption among women with a diagnosis of breast cancer and among cancer-free women. We used data from 23 420 women aged 50 -64 years who participated in the Diet, Cancer and Health cohort, of whom 449 were diagnosed with breast cancer between baseline (1993 -1997) and follow-up (2000 -2002), and 22 971 remained cancer-free. We used multiple linear regression analysis to examine differences in BMI and alcohol and tobacco consumption between the two groups and to examine whether demographic and prognostic factors were associated with behavioural changes in women with breast cancer. Results . There were no signifi cant differences in changes in BMI, alcohol and tobacco consumption between the two groups. Only in sub-analyses among women who lost weight between baseline and follow-up, women with breast cancer lost more weight than cancer-free women ( β ϭ 0.2; CI 0.1; 0.4), but residual confounding from stage cannot be excluded. Among the women with breast cancer we found no signifi cant changes in BMI, alcohol and tobacco consumption by level of education, marital status, chemotherapy, hormone therapy or radiation. Conclusion . Women with breast cancer did not reduce their BMI, or modify their alcohol use or tobacco consumption compared with cancer-free women. This study indicates that guidelines and interventions to change health behaviour are needed after a cancer diagnosis.Breast cancer is the most frequent cancer and the most frequent cause of death from cancer among women worldwide [1]. In Denmark, as in many other western countries, increasing breast cancer incidence and an improved fi ve-year survival rate of 85% [2] implies that a large proportion of women with breast cancer survive for many years beyond their initial treatment. It is therefore relevant to examine factors that could impact breast cancer survival and quality of life. Health behaviour including changes in body mass index (BMI), alcohol and tobacco consumption may be important not only for reducing breast cancer risk but also for survival [3,4] and quality of life [5], but knowledge on whether women with breast cancer do make more health behaviour changes compared to other women is limited.Most studies on health behaviour have addressed only changes in diet and physical activity [6] and most of the studies evaluating changes in alcohol and tobacco use and BMI were retrospective [5], therefore, possibly involving recall bias. We only identifi ed one prospective study that focused mainly on dietary change, but also included changes in BMI, alcohol and tobacco consumption following [7]. The authors found that women with breast cancer quit smoking more often than cancer-free wome...