From Jan. 1, 1983, through Dec. 31, 1987, 420 gravidas with insulin-requiring diabetes antedating pregnancy delivered on the Joslin Clinic service. Among them, 110 pregnancies (26.2% of the total) delivered before 37 completed weeks of gestation compared with a 9.7% incidence (906/9368) for the general population at the Brigham and Women's Hospital during calendar year 1985. Thirty-three percent of all premature deliveries were the result of the development of preeclampsia. The relative risk of prematurity for diabetic patients with any hypertensive complication was 2.0 (95% confidence interval, 1.40 to 2.87) compared with normotensive diabetic subjects. Compared with the general population, most of the excess risk of prematurity was confined to hypertensive diabetics and normotensive patients of more advanced White class. A history of having had a previous premature delivery, increasing duration of diabetes antedating pregnancy, and carrying a male fetus in the index pregnancy were significantly associated with premature delivery. Future efforts to reduce the incidence of prematurity among diabetic gravidas should be directed toward reducing the incidence of preeclampsia.