Background: Obesity and inactivity are linked to poor outcomes in breast cancer survivors. Recent reports suggest these factors may be especially important in young women with breast cancer, but relatively little is known about their physical activity (PA) and weight patterns in survivorship. Methods: The Young and Strong Study was a cluster-randomized trial evaluating the effect of an exportable education and support intervention for young breast cancer survivors. Sites were randomized 1:1 to a Young Women’s Intervention (YWI), focused on increasing attention to young women’s issues, or to a contact time control physical activity intervention (PAI). Women were enrolled within 3 months of diagnosis. PA (measured with the modified Godin Leisure Score Index) and self-reported height and weight were collected at baseline, and 3, 6, and 12 months after enrollment. Longitudinal analyses evaluated changes in PA and BMI over the entire study population, using non-parametric tests. Results: 467 patients enrolled between 7/12 -12/13 at 54 sites across the U.S. Median age at diagnosis was 40yrs (range 22-45). At baseline, median BMI was 27 kg/m2 and participants reported an average of 85 minutes of moderate/vigorous PA/wk. PA increased significantly over time (Table). At baseline, 52% of participants reported no PA, and at 12 months, only 31% reported no PA (P < 0.001). BMI increased modestly over the study period (increase of 3lbs, sd = 15); there was no relationship between change in BMI and PA or change in PA at any time point (all Spearman |rho| < 0.05). Conclusions: In this large cohort of young breast cancer survivors, more than 50% of study participants engaged in no PA at all in the months after breast cancer diagnosis, but PA increased over time. The majority of study participants were overweight or obese, and BMI increased modestly over time, despite increases in PA. These findings provide important information for the development of lifestyle interventions aimed at young breast cancer survivors. Clinical trial information: NCT01647607

* Baseline vs. 12-months.