Background Major abdominal oncology surgery is associated with substantial postoperative loss of functional capacity, and exercise may be an effective intervention to improve outcomes. The aim of this study was to assess efficacy, feasibility and safety of a supervised postoperative exercise programme. Methods We performed a single-blind, parallel-Arm, randomized trial in patients who underwent major abdominal oncology surgery in a tertiary university hospital. Patients were randomized to an early mobilization postoperative programme based on supervised aerobic exercise, resistance and flexibility training or to standard rehabilitation care. The primary outcome was inability to walk without human assistance at postoperative day 5 or hospital discharge. Results A total of 108 patients were enrolled, 54 into the early mobilization programme group and 54 into the standard rehabilitation care group. The incidence of the primary outcome was nine (16.7%) and 21 (38.9%), respectively (P=0.01), with an absolute risk reduction of 22.2% [95% confidence interval (CI) 5.9-38.6] and a number needed to treat of 5 (95% CI 3-17). All patients in the intervention group were able to follow at least partially the exercise programme, although the performance among them was rather heterogeneous. There were no differences between groups regarding clinical outcomes or complications related to the exercises. Conclusions An early postoperative mobilization programme based on supervised exercises seems to be safe and feasible and improves functional capacity in patients undergoing major elective abdominal oncology surgery. However, its impact on clinical outcomes is still unclear. Clinical trial registration NCT01693172. © The Author 2017. Published by Oxford University Press on behalf of the British Journal of Anaesthesia.All rights reserved.