The aim of the present study was to evaluate the effectiveness of Acceptance and Commitment Therapy (ACT) for treatment of anorexia nervosa (AN) using a case series methodology among participants with a history of prior treatment for AN. Three participants enrolled; all completed the study. All participants had a history of 1-20 years of intensive eating disorder treatment prior to enrollment. Participants were seen for 17-19 twice-weekly sessions of manualized ACT. Symptoms were assessed at baseline, post-treatment and 1-year follow-up. All participants experienced clinically significant improvement on at least some measures; no participants worsened or lost weight even at 1-year follow-up. Simulation modelling analysis (SMA) revealed for some participants an increase in weight gain and a decrease in eating disorder symptoms during the treatment phase as compared to a baseline assessment phase. These data, although preliminary, suggest that ACT could be a promising treatment for subthreshold or clinical cases of AN, even with chronic participants or those with medical complications.