Understanding the high rate of treatment adherence in trials of people with eating disorders is important as it can compromise the quality of the trials. In clinical practice, it may also contribute to illness chronicity, relapse, and costs. Thus, we investigated factors associated with adherence to a new treatment HAPIFED, which integrates cognitive behavioural therapy having extended sessions with body weight loss therapy compared to cognitive behavioural therapy with extended sessions alone, for individuals with Bulimia Nervosa or Binge Eating Disorder or other eating disorders comorbid with overweight or obesity.