Merrell reviews data concerning the role of heredity in manic-depressive psychosis. The literature confirms the cyclical nature of the disease and the fact that women are more often and more severely affected than men. As evidence for the important role of heredity in this illness, the author cites the significantly higher incidence of this psychosis in twins and among the relatives of persons with the disease, as well as the difference in frequency of the psychosis in different populations or in different areas.