This paper reports the preliminary results of a combined twin and family study of anorexia nervosa. Fifty-six per cent of the 25 female monozygotic (MZ) twin pairs and 5% of the 20 female dizygotic (DZ) twin pairs were concordant for anorexia nervosa. Nearly 5% of other female first degree relatives also had a history of anorexia nervosa. Analysis of data from the Eating Disorders Questionnaire (EDI) given to the twins and data as to weight loss, length of amenorrhoea and other characteristics of anorexia nervosa, together with the twin and family data, supports the hypothesis that genetic factors are very significant in the aetiology of anorexia nervosa. Analysis of this data using established techniques of psychiatric genetics suggested that up to 80% of the variance in liability to anorexia nervosa may be accounted for by genetic factors. The problems of this type of analysis are discussed as is the background to the genetics of weight and appetite control. A genetic/environmental model accounting for the features of anorexia nervosa is proposed.