Objective To investigate factors which predict positive treatment outcome in inpatients with anorexia nervosa (AN), particularly the role of early treatment response. Method 102 patients entering specialist inpatient treatment were assessed for eating disorder history, psychopathology, and motivation to change. Predictive factors assessed were: early treatment response defined as weight increase of at least 0.5–1 kg/week during the first 6 weeks of treatment (n=87), admission body mass index (BMI), onset age, chronicity, motivation to change, diagnosis, and previous hospitalization for AN. Positive treatment outcome was defined as achieving a BMI of 17.5 kg/m2 within an individual time frame. Results Logistic regression indicated that patients were 18 times more likely to reach positive treatment outcome if they met the National Institute for Health and Care Excellence weight guidelines within the first 6 weeks of hospitalization. Higher admission BMI was also found to predict positive treatment outcome. Discussion Higher entry BMI and early weight gain predict positive treatment outcome in individuals receiving specialist AN inpatient treatment. Copyright © 2016 John Wiley & Sons, Ltd and Eating Disorders Association.