We evaluated the association between short-term change in body mass index (BMI) and breast density during a 1yr weight-loss intervention (Manchester, UK). We included 65 premenopausal women (35-45yr, ≥7kg adult weight gain, family history of breast cancer). BMI and breast density (semi-automated area-based, automated volume-based) were measured at baseline, 1yr and 2yr after study entry (1yr post intervention). Cross-sectional (between-women) and short-term change (within-women) associations between BMI and breast density were measured using repeated-measures correlation coefficients and multivariable linear mixed models. BMI was positively correlated with dense volume between-women (r=0.41, 95%CI: 0.17,0.61) but less so within-women (r=0.08, 95%CI: -0.16,0.28). There was little association with dense area (between-women r=-0.12, 95%CI: -0.38,0.16; within-women r=0.01, 95%CI: -0.24,0.25). BMI and breast fat were positively correlated (volume: between r=0.77, 95%CI: 0.69,0.84, within r=0.58, 95%CI: 0.36,0.75; area: between r=0.74, 95%CI: 0.63,0.82, within r=0.45, 95%CI: 0.23,0.63). Multivariable models reported similar associations. Exploratory analysis suggested associations between BMI gain from 20yr and density measures (standard deviation change per +5kg/m2 BMI: dense area: +0.61 (95%CI: 0.12,1.09); fat volume: -0.31 (95%CI: -0.62,0.00)). Short-term BMI change is likely to be positively associated with breast fat, but we found little association with dense tissue, although power was limited by small sample size.