BACKGROUND: The prevalence of obesity is increasing at an alarming rate, as is the number of morbidly obese individuals (i.e. BMI>40) in our society. Bariatric surgery is considered an effective treatment for morbid obesity with promising results on weight control, quality of life and health. However, the success of bariatric surgery in terms of health outcomes varies largely, which might be related to changes in physical activity from pre- to post-surgery. PURPOSE: This study aims to determine whether pre- to post-surgery changes in physical activity were associated with health outcomes such as, excessive weight loss, VO2max, fat-free mass and quality of life up to two years after surgery. METHODS: 3879 post-bariatric patients were divided into three groups based on pre- to post-surgery change in physical activity: decrease, stable and increase. Measurements regarding physical activity (Baecke questionnaire), body composition (bioelectrical impedance analysis), cardiorespiratory fitness (Astrand) and quality of life (SF-36) were performed pre-surgery and two years post-surgery. Linear regressions between change in physical activity and change in health outcomes were conducted. RESULTS: Increasing physical activity was associated with larger excessive weight loss (β=3.17; 95%CI=1.40-4.93; P<0.001) and greater increases in VO2max (β=2.01; 95%CI=0.51-3.51; P=0.009) and %fat-free mass (β=1.05; 95%CI=0.50-1.60; P<0.001) compared to stable- and declining physical activity. Decreasing physical activity was associated with a decrease in VO2max (β=-3.91; 95%CI=-6.40- -1.43; P=0.002). The increase-group showed greater increases in all quality of life subscales compared to stable- and decrease-group (P<0.05), except for physical functioning. Change in physical activity was not related to changes in absolute fat-free mass. CONCLUSIONS: Increasing physical activity from pre- to post-surgery was associated with greater excessive weight loss and greater improvements in body composition, cardiorespiratory fitness and quality of life. Therefore, increasing physical activity after bariatric surgery seems essential for bariatric success in terms of health outcomes.