BackgroundThe superiority of Enhanced Cognitive Behavior Therapy (CBT-E) with regard to weight gain and improvement of psychopathology of eating disorders for patients with anorexia nervosa (AN) over other psychotherapies and treatment as usual (TAU) has not been demonstrated in randomized controlled trials (RCTs). However, a previous RCT showed that patients with AN whose baseline body mass index (BMI) was less than 17.5 kg/m2gained more weight when treated with CBT-E than with other psychotherapies. The aim of the study is to compare the efficacy of CBT-E and TAU for patients with AN. It was hypothesized that CBT-E would be superior to TAU, at least in terms of weight gain, as most patients with AN are likely to have a BMI lower than 17.5 kg/m2.Methods/designA randomized parallel-group multicenter trial will be conducted in three teaching hospitals in Japan between January 2023 and March 2026. Patients with DSM-5 AN, aged 16 years and older, with a BMI equal to or above 14.0 and below 18.5 will be eligible to participate. 56 patients will be randomly and evenly assigned to two intervention groups (CBT-E and TAU). Those assigned to CBT-E will be offered 25–40 sessions in accordance with their initial BMI. Patients assigned to TAU will have at least one session every 2 weeks, with the number of sessions and treatment period not fixed in advance. The primary outcome is BMI at 40 weeks after treatment initiation. The secondary outcomes are the results from the Japanese version of the Eating Disorder Examination Questionnaire and Clinical Impairment Assessment questionnaire to measure eating disorder psychopathology and psychological impairment. The follow-up assessment will be performed 6 months after the 40-week assessment.DiscussionThis multi-center randomized controlled study will probably evaluate the efficacy of CBT-E compared with TAU for patients with more severe AN than in previous studies since Japanese patients are likely to have a lower BMI than those in Western countries. While it may be difficult to generalize the results of a study conducted in Japan, it would be valuable to clarify the efficacy of CBT-E as a treatment package.Trial registrationUMIN, UMIN000048847. Registered 12 Sep 2022.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13030-023-00277-2.