Assessing clinically meaningful change is valuable for treatment planning, monitoring course of illness and evaluating outcome. Although DSM eating disorder (ED) diagnoses have been criticized for poor clinical utility, instability, and uncertainty, remission/change of diagnosis is often the standard for evaluating outcome. We tested the validity of the clinically significant reliable change index (CS/RCI) compared to change in DSM-IV ED-diagnoses. We investigated if CS/RCI was concordant to diagnostic change and compared explained variance on measures at follow-up. Using a database for specialized ED treatment in Sweden the sample contained 1042 female patients (246 adolescents/796 adults). CS/RCI was calculated for the Clinical Impairment Assessment (CIA) and the Eating Disorder Examination Questionnaire (EDE-Q). CS/RCI explained more variance in gain scores for psychopathology measures than diagnostic change (DSM-IV). Average agreement between diagnostic change and CS/RCI was 62% and 60% for CIA and EDE-Q, respectively. Diagnostic change always resulted in more positive outcome than CS/RCI. Together with clinical judgment, CS/RCI is a valuable method for determining clinically significant changes in clinical practice and research. It is economically sound and results are easily interpreted and communicated to patients.