In a sample of 38 eating disorder (ED) patients who received psychotherapeutic treatment, changes in attachment security, and mentalization in relation to symptoms reduction were investigated. Attachment security improved in 1 year but was unrelated to improvement of ED or comorbid symptoms. Mentalization did not change significantly in 1 year. Pretreatment mentalization was negatively related to the severity of ED symptoms, trait anxiety, psycho-neuroticism, and self-injurious behavior after 1 year of treatment. We conclude that for ED patients, improving mentalization might increase the effect of treatment on core and comorbid symptoms.