SummaryIn three hospitals 81 female patients satisfying rigorous diagnostic criteria for anorexia nervosa were randomly allocated to one of four treatment combinations of cyproheptadine and placebo with behaviour therapy and no behaviour therapy. Cyproheptadine was found to be effective in inducing weight gain in a subgroup of anorexia nervosa patients who (a) had a history of birth delivery complications, (b) had lost 41–52 per cent weight from norm and (c) had a history of prior outpatient treatment failure. This subgroup may represent a more severe form of anorexia nervosa.