We investigated pre- to post-diagnosis weight change and functional limitations in a cohort of breast cancer survivors. A cohort of 1,841 early-stage breast cancer survivors provided information on pre- and post-diagnosis weight and physical function on average 2 years post-diagnosis. The mean number of limitations for each BMI category and each weight change category were compared using the Wilcoxon test. Cross-sectional associations between weight change, from 1 year prior to diagnosis to 2 years post-diagnosis, and functional limitations were determined using logistic regression. Women with BMI???30 kg/m(2) had significantly higher physical limitations compared to women with BMI?25 kg/m(2) (2.06 vs 0.96 for moderate/severe limitations, 3.92 vs 3.27 for mild limitations, 1.31 vs 0.47 for lower body limitations, and 0.76 vs 0.49 for all other limitations; P?0.0001). Women who reported a large weight gain (?10% of pre-diagnosis weight) were more likely to report any limitation (OR?=?1.79; 95% confidence interval (CI)?=?1.23-2.61), a moderate/severe limitation (OR?=?2.30; 95% CI?=?1.75-3.02), and a lower body limitation (OR?=?2.05; 95% CI?=?1.53-2.76) compared to women who maintained weight within 5% of pre-diagnosis weight. However, associations between weight loss and functional limitations depended on pre-diagnosis BMI and comorbidity status. Among women without comorbidity, large weight loss (?10% of pre-diagnosis weight) in normal-weight women was associated with higher risk of functional limitations, whereas among overweight/obese women, large weight loss appeared to be associated with a lower risk of limitations. Among women with comorbidity, moderate weight loss in overweight/obese women was associated with a higher risk of a moderate/severe physical limitation. Large weight gain was associated with a higher risk of physical functional limitations, but associations between weight loss and functional limitations may depend on initial BMI and comorbidity status. In this study we found that both weight loss and weight gain among breast cancer survivors were associated with a higher risk of physical functional limitations. Weight maintenance, therefore, may be an important factor in preventing and/or reducing the risk of functional decline in breast cancer survivors.