Background: There is ample evidence on the efficacy of different therapeutic approaches in treating eating disorders. Yet, studies on the effectiveness of inpatient/day-hospital treatment of eating disorders are rare. Method: Data were retrieved from a naturalistic multi-center effectiveness study of inpatient and day-hospital treatment in departments of psychosomatic medicine and psychotherapy in Germany (MEPP). N = 151 patients with Anorexia Nervosa (AN) and atypical AN, as well as n = 120 patients with Bulimia Nervosa (BN) and atypical BN were included. Eating disorder pathology (EDE-Q) and BMI were assessed on admission, at discharge, and 1 year after discharge. Changes over time were analysed in both outcomes. Treatment success was assessed individually using reliable change (RC) indices. Regression analyses were used to identify predictors of RC. Results: EDE-Q scores decreased in both patient groups. One year after discharge, RC was estimated in 33.8% of patients with AN and 43.3% of patients with BN. 24.2% of patients with AN and 54.3% with BN showed partial remission. Discussion: Results from this multi-center effectiveness study are comparable to those of former national and European studies. Based on the results of individual treatment success, recommendations for inpatient/day-hospital treatment can be derived. Trial Registration: The study was registered at the German Clinical Trials Register (www.drks.de; ID: DRKS00016412).