CONTEXT: Weight loss using low-calorie diets produces variable results, presumably due to a wide range of energy deficits and low-dietary adherence. OBJECTIVE: Our objective was to quantify the relationship between dietary adherence, weight loss, and severity of caloric restriction. DESIGN AND SETTING: Participants were randomized to diet only, diet-endurance training, or diet-resistance training until body mass index (BMI) was less than 25 kg/m(2). PARTICIPANTS: Healthy overweight (BMI 27-30) premenopausal women (n = 141) were included in the study. INTERVENTIONS: An 800-kcal/d(-1) diet was provided, and the exercise groups were engaged in three sessions per week. MAIN OUTCOMES: Dietary adherence, calculated from total energy expenditure determined by doubly labeled water measurements and dual-energy x-ray absorptiometry body composition changes, and degree of caloric restriction were determined. RESULTS: All groups had similar weight loss (approximately 12.1 +/- 2.5 kg) and length of time to reach target BMI (approximately 158 +/- 70 d). Caloric restriction averaged 59 +/- 9%, and adherence to diet was 73 +/- 34%. Adherence to diet was inversely associated to days to reach target BMI (r = -0.687; P < 0.01) and caloric restriction (r = -0.349; P < 0.01). Association between adherence to diet and percent weight lost as fat was positive for the diet-endurance training (r = 0.364; P < 0.05) but negatively correlated for the diet-only group (r = -0.387; P < 0.05). CONCLUSIONS: Dietary adherence is strongly associated with rates of weight loss and adversely affected by the severity of caloric restriction. Weight loss programs should consider moderate caloric restriction relative to estimates of energy requirements, rather than generic low-calorie diets.