We are convinced that the problem of heart diseases complicating pregnancy is not generally understood. Though nearly 20 per cent of the maternal deaths at the Boston Lying-In Hospital in a four year period and 28 per cent of the maternal deaths at the Faulkner Hospital, Boston (private obstetric wing), in a seven year period were furnished by patients with seriously injured hearts, only about 1 per cent of all pregnant patients have seriously injured hearts. This hinders obstetricians from personally observing large numbers of cardiac cases and masks their importance. The following brief outline of the principal points in the problem is based on seven years' intensive study of heart diseases in pregnant women at the Boston Lying-In Clinic, and on our experience in private cases. (The statements we make dogmatically could be supported by case analyses were there space. When we necessarily speak from opinion, we will say