During the past decade, substantial progress has been made in understanding and managing the pregnant diabetic patient and her fetus. Currently, with proper attention to metabolic control, the perinatal mortality for pregnancies complicated by diabetes, excluding lethal congenital anomalies, should be no higher than for the general population. Of all complications that could occur in the antenatal management of the diabetic, ketoacidosis is perhaps the most feared. The mortality for nonpregnant adults in the United States in 1975 approached 10% with one episode of ketoacidosis. Fetal mortality with this complication has in some series has been as high as 90%. In this article we discuss the pathophysiology, diagnosis, management, and prevention of ketoacidosis in pregnancy complicated by diabetes. Metabolic alterations in carbohydrate and fat metabolism that are found in pregnancy and that predispose the pregnant diabetic to develop ketoacidosis are reviewed.