In this paper we have presented five cases of mental abnormality of various types in one identical twin. We have attempted to show that because of the identical heredity in monozygotic twins the study of cases of this type is of importance in showing the relative rôle played by inherited and environmental factors. In four cases we have been able to determine differences in environment which we feel are important in the etiology of their condition, knowing that by the continued mental health of the other twin we have ruled out the possibility that heredity in itself was the factor behind the illness. If we look upon studies of abnormality of one identical twin as a testing ground for our present day ideas on etiology, then our results are quite in keeping with these conceptions.Abstract Teaser