Identical twin girls, whose monozygosity was determined by blood group analysis, simultaneously developed anorexia nervosa at the age of eleven. Treatment consisting of individual, outpatient, insight-oriented psychotherapy for each girl by separate therapists, and work with the parents by a social worker, uncovered significant conflict in the area of identity formation, apparently exacerbated by the twinship. Resolution of the anorexia and significant ego growth in both twins rapidly followed exploration of the twinship dynamics in therapy. Gains made during the relatively brief therapy were well maintained at a two-year follow-up evaluation.