Background: While eating disorder not otherwise specified (EDNOS) is the most common eating disorder (ED) diagnosis in routine clinical practice, no specific treatment methods for this diagnosis have yet been developed and studied. Enhanced cognitive behavioral therapy (CBT-E) has been described and put to the test as a transdiagnostic treatment protocol for all EDs, including EDNOS. Initial research in the UK suggests that CBT-E is more effective for EDs, especially bulimia nervosa (BN) and EDNOS, than the earlier version of CBT. These positive results of CBT-E have to be replicated in more detail, preferably by independent researchers in different countries. Being the first Dutch study into CBT-E, the results from this national multicenter study - on three sites specialized in EDs - will deliver important information about the effectiveness of CBT-E in several domains of ED pathology, while providing input for the upcoming update of the Dutch Multidisciplinary Guideline for the Treatment of Eating Disorders. Methods/design: A multicenter randomized controlled trial will be conducted. One hundred and thirty-two adult outpatients (aged 18 years and older) with an ED diagnosis and a Body Mass index (BMI) of between 17.5 and 40 will be randomly allocated to the control or the intervention group. Subjects in the control group will receive Treatment as Usual (standard outpatient treatment provided at the participating sites). Subjects in the intervention group will receive 20 sessions of CBT-E in 20 weeks. The design is a 2 (group) x 5 (time) repeated measures factorial design in which neither therapists nor patients will be blinded for treatment allocation. The primary outcome measure is recovery from the ED. Secondary outcome measures include ED psychopathology, common mental disorders, anxiety and depressive symptoms, health-related quality of life, health care use and productivity loss. Self-esteem, perfectionism and interpersonal problems will be examined as putative predictors and mediators of the effect of treatment. Also, an economic evaluation from a societal perspective will be undertaken. All relevant effects, direct and indirect costs will be included. Utility scores will measure the effects. Measurements will take place at pretreatment, 6 weeks, 20 weeks, 40 weeks and 80 weeks. Discussion: This effectiveness study into CBT-E has the aim of broadening the scope and generalizability of former studies. If CBT-E appears to be at least as effective as traditional diagnosis-specific treatments for a broad range of ED patients, training in one protocol would be sufficient for clinicians to treat patients with different kinds of EDs. It gives the opportunity to offer treatment for a severe mental disorder with fewer resources, thereby increasing the accessibility of specialized care for patients with an ED.