Hemoglobin A1c (HgbA1c) levels were determined within 24 hours after delivery in 88 nondiabetic and 73 diabetic women belonging to White's classes A to D. Diet-controlled gestational diabetic women had mean (+/- SE) Hgb A1c levels similar to those of nondiabetics (5.8 +/- 0.18% vs 5.7 +/- 0.08%). Mean Hgb A1c levels were higher in insulin-requiring gestational (6.4 +/- 0.20%, P less than .05) and permanent (6.5 +/- 0.27%, P less than .05) diabetics than in nondiabetics. The proportions of subjects with levels above the normal range were also larger in the insulin-requiring groups. Mean Hgb A1c levels and the proportions of abnormally high levels were similar for mothers of macrosomic and of normally grown neonates in the nondiabetic as well as in the various diabetes groups. There was no correlation between maternal Hgb A1c level and neonatal birth weight, either real or relative. There were also no statistically significant differences in mean Hgb A1c levels between mothers of neonates with or without hypoglycemia within four hours of birth. Hgb A1c measurement did not permit differentiation between those mothers of macrosomic neonates who were diabetic and those who were not. In conclusion, although Hgb A1c level has been shown to reflect diabetic control, our data suggest that it may not be reliable as an indicator of fine tuning during the third trimester of pregnancy or as a predictor of the effects of diabetes on the fetus.