Serial measurements of the HbA1c levels were performed during pregnancy in 4 groups of patients attending Antenatal Clinics: 36 normal pregnancies; 16 pregnancies in established insulin-dependent diabetic patients; 9 patients with gestational diabetes diagnosed during that pregnancy; and 21 patients who had been diagnosed as having gestational diabetes in at least one previous pregnancy. In the normal pregnancy HbA1c levels showed a small but significant increase from the end of the first trimester to delivery despite blood glucose levels remaining constant throughout. In the insulin-dependent and gestational diabetic patients, blood glucose levels remained significantly higher than in the normal throughout pregnancy but only in insulin-dependent diabetic patients and the newly diagnosed untreated gestational diabetic patients were the HbA1c levels significantly higher than in the normal. In those patients who had previous pregnancies complicated by gestational diabetes, blood glucose levels were significantly higher than in the normal but HbA1c levels were not. This dissociation between blood glucose and HbA1c levels in gestational diabetic pregnancies in particular limits the value of HbA1c levels in monitoring antidiabetic treatment in such pregnancies.