To examine physical activity (PA) patterns from pre- to post-diagnosis, and compare these changes to women without breast cancer. To determine pre-diagnosis predictors of PA change, post-diagnosis, in breast cancer survivors (BCS). Data were from 2314 Study of Women’s Health Across the Nation (SWAN) participants, average age of 46.4 ± 2.7 years at baseline (1996-1997). In Pink SWAN, 151 women who reported an incident breast cancer diagnosis over 20 years were classified as BCS; the remaining 2163 women were controls. LOESS plots and linear mixed models were used to illustrate and compare PA changes (sports/exercise [primary measure] and total PA) from pre- to post-diagnosis (or corresponding period) in BCS versus controls. Adjusted linear regression models were used to determine pre-diagnosis predictors of at-risk post-diagnosis PA change patterns (consistently low and decreased PA). No differences in pre- to post-diagnosis PA (or corresponding period) were observed in BCS versus controls. Among BCS, the odds of at-risk post-diagnosis PA change patterns was 2.50 (95% CI 0.96-6.48) times higher for those who reported sleep problems at ≥ 50% (compared to 0%) of pre-diagnosis visits and 3.49 (95% CI 1.26-9.65) times higher for those who were overweight or obese at all (compared to no) pre-diagnosis visits. No other statistically significant predictors were noted. Age-related declines in PA were not amplified by a breast cancer diagnosis. Given the beneficial role of PA across the cancer control continuum, efforts to increase or maintain adequate PA, post-diagnosis, should be continued. While age-related physical activity declines were not amplified breast cancer diagnosis, efforts to identify breast cancer survivors at increased risk for post-diagnosis physical activity declines (or maintenance of low activity) may be a high-yield strategy to improve prognosis and quality of life.