Glycosylated hemoglobin (HbA1) was determined in three subject groups: 69 non-diabetic mothers who were delivered of normal weight infants at term (Group I), 33 non-diabetic mothers who were delivered of macrosomic infants (greater than 4000 g) at term (Group II), 51 gestational diabetics in the 3rd trimester--before onset of the diabetes therapy (Group III). In all three groups diagnostic assessment of glucose regulation was done by means of the oral glucose tolerance test during the 3rd trimester. Glycosylated hemoglobin was assayed by cation-exchange chromatography in small disposable columns. The mean values and standard deviations of HbA1 were 6.51 +/- 0.46% in Group I, 6.59 +/- 0.42% in Group II and 7.11 +/- 0.56% in Group III. Between the HbA1 values of Group III (gestational diabetes) on the one hand and those of the non-diabetic groups I and II on the other, there were highly significant differences (p less than 0.001; x2-test). HbA1 values above 7.4%--i.e. above mean + 2 s. d. of HbA1 in the non-diabetic mothers--were with 95% probability abnormal and indicative of gestational diabetes. HbA1 values between 7.0% and 7.4% were suspected of impaired glucose tolerance and gestational diabetes respectively. Between the HbA1 levels in the non-diabetic groups and those in the gestational diabetic group there was a vast zone with overlapping values. HbA1 data situated in this transitional area could be found both in non-diabetic subjects and also in those with abnormal glucose regulation. HbA1 values below 6.0% excluded gestational diabetes or otherwise impaired glucose tolerance with a high degree of probability.