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Observational studies suggest that physical activity after a breast cancer diagnosis is associated with improved cancer outcomes; however, no randomized data are available. Here, we report an exploratory follow-up of cancer outcomes from the Supervised Trial of Aerobic versus Resistance Training (START).The START was a Canadian multicenter trial that randomized 242 breast cancer patients between 2003 and 2005 to usual care (n = 82), supervised aerobic (n = 78), or resistance (n = 82) exercise during chemotherapy. The primary end point for this exploratory analysis was disease-free survival (DFS). Secondary end points were overall survival, distant DFS, and recurrence-free interval. The two exercise arms were combined for analysis (n = 160), and selected subgroups were explored.After a median follow-up of 89 months, there were 25/160 (15.6%) DFS events in the exercise groups and 18/82 (22.0%) in the control group. Eight-year DFS was 82.7% for the exercise groups compared with 75.6% for the control group (HR, 0.68; 95% confidence interval (CI), 0.37–1.24; log-rank, P = 0.21). Slightly stronger effects were observed for overall survival (HR, 0.60; 95% CI, 0.27–1.33; log-rank, P = 0.21), distant DFS (HR, 0.62; 95% CI, 0.32–1.19; log-rank, P = 0.15), and recurrence-free interval (HR, 0.58; 95% CI, 0.30–1.11; Gray test, P = 0.095). Subgroup analyses suggested potentially stronger exercise effects on DFS for women who were overweight/obese (HR, 0.59; 95% CI, 0.27–1.27), had stage II/III cancer (HR, 0.61; 95% CI, 0.31–1.20), estrogen receptor-positive tumors (HR, 0.58; 95% CI, 0.26–1.29), human epidermal growth factor receptor 2-positive tumors (HR, 0.21; 95% CI, 0.04–1.02), received taxane-based chemotherapies (HR, 0.46; 95% CI, 0.19–1.15), and ≥85% of their planned chemotherapy (HR, 0.50; 95% CI, 0.25–1.01).This exploratory follow-up of the START provides the first randomized data to suggest that adding exercise to standard chemotherapy may improve breast cancer outcomes. A definitive phase III trial is warranted.