Background: Postdiagnosis weight gain in non-familial breast cancer (BC) survivors has been observed in a number of studies, with the reported prevalence and amount of weight gain being highly variable. Most studies did not include a cancer-free comparison group. Change in BMI has not been studied in BC survivors from high-risk populations. Methods: In an ongoing prospective cohort of women at Johns Hopkins with a family history of BC, ovarian cancer, and/or a BRCA1/2 mutation, we identified 249 survivors of stage 0-III BC and 400 cancer-free women who completed a baseline and at least one follow-up questionnaire and were age > 30 years (yrs) at enrollment. Linear regression was used to estimate % change in self-reported BMI from baseline to follow-up (median 3.7 yrs) for survivors compared to cancer-free women including the effects of time between BC diagnosis and baseline, and differing BC treatment. Results: Mean age at baseline was 54.6 yrs among for survivors and 50.2 yrs among for cancer-free women. Mean age at BC diagnosis was 48.7 yrs with > 50% diagnosed under 50 yrs, and 23% with ER- tumors. Baseline BMI did not differ between survivors and cancer-free women irrespective of tumor subtype. However, the average change in BMI was 1.79% (95% CI 0.32, 3.26) greater in survivors compared to cancer-free women in models adjusted for age, baseline BMI, menopausal status, physical activity, enrollment yr, and bilateral oophorectomy. The increase in BMI was highest (β = 4.22%; 95% CI 1.74, 6.70) in survivors diagnosed within 1 yr of baseline. Change in BMI in survivors diagnosed > 5 yrs prior to baseline did not significantly differ from cancer-free women. A significantly greater increase in BMI was also observed in survivors diagnosed at age > 50 yrs, and in those who received chemotherapy and not hormone therapy. Conclusions: In this study of BC survivors and cancer-free women enrolled from the same source population, we observed a greater percent increase in BMI over time among survivors, particularly those diagnosed within 5 yrs of baseline and/or treated with chemotherapy. Survivors in these categories may benefit from interventions aimed to reduce weight gain post diagnosis.