Background: There is strong interest in testing lifestyle interventions to improve cancer outcomes, however, the optimal methods for achieving behaviour change in large scale pragmatic trials are unknown. We report the 1 year feasibility results for behaviour change in NCIC CTG CO.21 Colon Health and Life-Long Exercise Change (CHALLENGE) Trial. Methods: Between 2009 and 2014, 273 high risk stage II and III colon cancer survivors who had recently completed chemotherapy at 50 centers in Canada and Australia were randomized to a structured exercise program (SEP n = 136) or health education materials (HEM n = 137). The primary feasibility outcome in an a priordefined interim analysis was a difference between groups of ≥ 5 metabolic equivalent task (MET) hours/week in self-reported physical activity (PA) after at least 250 participants had reached 1 year. Secondary feasibility outcomes included cardiorespiratory fitness, body weight and circumferences, and objective physical functioning. Results: The SEP group reported a mean increase from baseline to 1 year PA of 15.6 MET hours/week vs a mean increase of 5.1 in the HEM group (+10.5 hrs/wk [95% CI = +3.1 to +17.9]; p = 0.002). SEP also improved relative to HEM in mean differences for predicted VO2max (+2.2ml/kg/min; p = 0.068), 6 minute walk (+29m; p < 0.001), 30 second chair stand (+1.6reps; p < 0.001), 8 foot up and go (-0.4s; p = 0.004), and the sit and reach (+2.1cm; p = 0.08). Conclusions: The behaviour change intervention in the CO.21 Trial produced an increase in self-reported PA that exceeded the feasibility criterion for trial continuation, resulted in objective fitness improvements, and is consistent with the amount of PA associated with improved colorectal cancer outcomes in observational studies. Recruitment to CO.21 remains ongoing. Clinical trial information: NCT00819208.