PURPOSE: Improving cardiopulmonary reserve, or peak oxygen consumption (VO2peak) measured during cardiopulmonary exercise testing (CPET), may reduce complications after surgery. This feasibility study determined the effectiveness of a supervised, preoperative High Intensity Interval Training (HIIT) program in increasing VO2peak by 2ml/kg/min. Clinical outcomes were documented to determine the endpoint most sensitive to improved fitness. METHODS: In this prospective study, participants aged 50-85 undergoing major abdominal surgery were randomised to standard care or 14 sessions of HIIT over 4-6 weeks. HIIT sessions involved approximately thirty minutes of stationary cycling (5 warm-up, 20 interval training, 5 cool-down). Interval training alternated 1 minute of high and low intensity pedalling, with the goal of reaching 90% maximum heart rate during the session. Clinical outcomes included complications, postoperative morbidity survey, length of stay, and Short Form-36 quality of life questionnaire (SF-36). RESULTS: Of 63 participants, 46 completed both CPETs and 51 completed clinical follow-up. On per protocol analysis, mean VO2peak increased 14%, from 20.3-23.2ml/kg/min in the exercise group and 0.7%, from 21.8-22.0 ml/kg/min in the control group. Change in VO2peak was 2.87 vs. 0.14 ml/kg/min (p<.001). Exercisers increased peak work rate by 25 watts. Although there were no significant differences in the clinical outcomes, those most responsive to improved fitness with exercise were: the total number of postoperative complications (0.64 in exercisers vs. 1.16 in controls per patient, p=.07) and the physical component score of the SF-36 (p=.07), with the greatest difference six weeks after surgery indicating a quicker postoperative recovery. CONCLUSION: There was a significant improvement in VO2peak and peak work rate with preoperative HIIT. We also noted a trend towards fewer postoperative complications and a more rapid recovery after surgery. Preoperative HIIT results in a clinically important improvement in cardiopulmonary reserve for patients undergoing abdominal surgery.