Objective: To measure adherence to a specific exercise prescription (1500 kcal week-1) by objectively quantifying unsupervised exercise energy expenditure (ExEE) in obese women. Design: The 16-week lifestyle intervention consisted of weekly meetings with research staff and promotion of increased ExEE (1500 kcal week-1) and a decreased dietary intake (-500 kcal day-1). Participants: Twenty-nine obese females (body mass index=36.8±5.0 kg m-2, body fat=49.6±3.7%) from a hospital-based lifestyle intervention were included in the analysis. Measurements: ExEE was estimated and monitored weekly using heart rate monitoring, and body composition was measured before and after the intervention by dual-energy X-ray absorptiometry. Results: Free-living adherence to the exercise prescription was variable and, on average, modest such that 14% achieved 1500 kcal week-1, and the average weekly ExEE (768 kcal week-1) represented 51.2% of the total amount prescribed. ExEE was correlated with changes in body weight (r=0.65, P<0.001) and fat mass (r=0.65, P<0.001). Achievement of a 5% weight loss target was dependent on the achievement of an ExEE level of 1000 kcal week-1 (P<0.001). Exercise 'adherers' (>1000 kcal week-1) lost more weight (-9.9 vs -4.1 kg), more fat mass (-6.8 vs -3.0 kg) and more waist circumference (-9.8 vs -5.6 cm) when compared to 'non-adherers' (<1000 kcal week-1). Discussion: Exercise is an integral component of lifestyle interventions aimed at reducing obesity and its complications. However, without accurate and objective measures of ExEE, it is difficult for relationships between exercise and health outcomes to be elucidated. The present study suggests an alternative to self-report to increase the confidence with which conclusions are drawn regarding the role of exercise within lifestyle interventions.