The relationship between the level of hemoglobin A1 (Hb A1) in the first trimester and major malformations and spontaneous abortions was examined in 303 insulin-requiring diabetic gravidas. During the study period, all patients with insulin-requiring diabetes mellitus antedating pregnancy who registered for prenatal care prior to 12 weeks' gestation and who had a known outcome were included. Thirty-five percent of the patients entered with a first-trimester Hb A1 of greater than 11.0% of total hemoglobin (9 standard deviations above the mean for a nondiabetic population). A broad spectrum of glycemic control was therefore represented. The risk of spontaneous abortion was 12.4% with first-trimester Hb A1 less than or equal to 9.3% and 37.5% with Hb A1 greater than 14.4% (risk ratio 3.0; 95% confidence interval 1.3-7.0). The risk for major malformation was 3.0% with Hb A1 less than or equal to 9.3% and 40% with Hb A1 greater than 14.4% (risk ratio 13.2; 95% confidence interval 4.3-40.4). Although the risks for both adverse outcomes were markedly elevated following a first trimester in very poor metabolic control, there was a broad range of control over which the risks were not substantially elevated. To keep malformations and spontaneous abortions to a minimum among diabetic women does not require "excellent" control; there seems to be a fairly broad range of "acceptable" control.