The influence of early pregnancy glycemic control as measured by hemoglobin A1c concentration and the incidence of congenital anomalies and spontaneous abortions were evaluated in women presenting for prenatal care with insulin-treated diabetes in a population whose glycemic control was poor. Thirty-one abnormal outcomes were seen in 83 pregnancies (37%). There were 22 spontaneous abortions and nine major congenital anomalies. No woman with an early pregnancy hemoglobin A1C value less than 9.5% had an infant with a congenital anomaly and a single woman experienced a spontaneous abortion (4%). Conversely, in women with an early pregnancy hemoglobin A1C value greater than or equal to 9.5%, congenital anomalies occurred in 24% and spontaneous abortion in 35%. Outcomes of pregnancies in type 1 and type 11 diabetic women were comparable. A strong statistical relationship between hemoglobin A1C and adverse pregnancy outcomes was demonstrated. These results strongly suggest that poor glycemic control during early pregnancy adversely influences pregnancy outcomes; the greater the degree of poor control, the greater the impact on pregnancy outcome. The data further justify the need for preconceptional control in diabetic woman and for careful evaluation of the fetus during pregnancy in the woman with insulin-treated diabetes.