In a collaborative study between St George's and the Maudsley Hospitals, 34 pairs of twins and one set of triplets were identified in which the proband had anorexia nervosa. The diagnosis was based on standard criteria and zygosity was established by blood group analysis or by use of the physical resemblance questionnaire. In the 30 female twin pairs, 9/16 of the monozygotic (MZ) and 1/14 of the dizygotic (DZ) pairs were concordant for anorexia nervosa. Three of the non-anorexia co-twins had other psychiatric illnesses, and two had minor eating disorders. None of the male co-twins had anorexia nervosa.