Objective: To test the hypothesis that women with insulin-dependent (type I) diabetes have a threshold of glycemic control in early pregnancy for increased risks of spontaneous abortion and congenital malformations.Methods: Receiver-operating characteristic (ROC) curves were formed for the occurrence of abortion and malformations as a function of the median first-trimester preprandial blood glucose concentration and the first measured glycohemoglobin concentration in pregnant women with type I diabetes.Results: Fifty-two of the 215 women (24%) who enrolled before 9 weeks' gestation had spontaneous abortions. Six percent of the women enrolled before 14 weeks had infants with major congenital malformations. Thresholds for an increased risk of abortion and malformations were a median first-trimester blood glucose concentration of 120–130 mg/dL or an initial glycohemoglobin concentration of 12–13% (6.2–7.5 standard deviations above the normal mean).Conclusions: Type I diabetic women with initial glycohemoglobin concentrations in pregnancy above 12% or median first-trimester preprandial glucose concentrations above 120 mg/dL have an increased risk of abortion and malformations. Below these glycemic thresholds, the risks are comparable to those in nondiabetic women.