BACKGROUND AND PURPOSE: Obese adults are at an increased risk for mobility-related problems. National guidelines recommend calorie restrictions and exercise for obese adults as a means to improve functional fitness capacity and to increase mobility. Yet, lifestyle weight loss interventions often fail to measure fitness changes. The aim of this study was to assess whether a 1-month, intensive behavioural change, diet and fitness intervention for overweight and obese adults would result in statistically significant and clinically meaningful changes in functional exercise. METHODS: A pre-post test design was used in this study. Seventy-two participants (40 women, 32 men; mean baseline body mass index (BMI) = 42.6 + 9.0; mean age = 45.8 + 16.8) completed a modified 6-minute walk test (6MWT), performed on a treadmill, at baseline and at end of treatment. RESULTS: Significant improvements included decreased BMI (2.7 + 1.7 kg m(-2) , p < 0.001) and increased 6MWT distance (66.4 + 73.0 m, p < 0.001). The 6MWT improved by 66 m on average, a reported clinically meaningful difference. Greater improvements in the 6MWT were significantly correlated with greater weight loss and BMI reduction. DISCUSSION: Our findings suggest that rehabilitation beyond weight loss may be derived from participation in a brief, intensive behavioural change, diet and fitness programme. Physiotherapists are in a prime position to address the physical and motivational challenges participants face while living with severe obesity: targeting functional exercise capacity is one key strategy for addressing immobility associated with obesity. Copyright © 2015 John Wiley & Sons, Ltd.