The American Cancer Society estimates that 1 in 8 women will be diagnosed with breast cancer in her lifetime. Due to 5-year survival rates of 89.7%, it is important to examine factors that impact physical function over the course of cancer treatment. Purpose: To prospectively analyze changes in physical function and the extent to which physical activity predicted these changes in a group of 400 women with recently diagnosed, staged breast cancer. Methods: Three visits were included in this study: pre-surgery, one-year, and two-years post-surgery. Each session consisted of assessments of physical activity (PA), perceived function, strength, shoulder mobility. Results: The women in this study were a mean age of 57 years, overweight with a mean BMI of 29.4 kg/m2, physically inactive, and diagnosed with early stage breast cancer. Linear regression analyses, controlling for age, BMI, and treatment, indicated that baseline PA predicted year one bicep strength (beta = .254, p < 0.001) and grip strength (beta = .146, p < 0.001) and year two bicep strength (beta = .304, p = 0.022). Conclusions: Since PA levels before and during treatment are below national recommendations and pre-treatment PA affects physical function, future research should target PA interventions concurrent with treatment to preserve function during survivorship.