Serial plasma diamine oxidase (DAO) titers and urinary pregnanediol and estriol excretions were measured in 41 pregnancies complicated by diabetes mellitus. Thirty-six per cent of the plasma enzyme titers were abnormally low in the Class A and 68 per cent in the combined Class B, C, and D diabetic pregnancies. All infants in the Class A diabetic group were born alive while there were 4 stillbirths in the Class B, C, and D diabetic patients. In each pregnancy resulting in a stillbirth, the plasma DAO curves were abnormally low from early pregnancy. Urinary pregnanediol excretions were within the normal range in each of the cases studied. In common with plasma DAO, urinary estriol appeared to correlate with the severity of the diabetic state. However, as with the plasma enzyme, abnormally low urinary estriol excretion was frequently associated with live births. It is our concept that low plasma DAO pregnancies represent fetuses at continuous "high risk".