In spite of dietary treatment, the infants of pregnant patients with abnormal glucose tolerance have hyperinsulinism and diabetogenic fetopathy in 10 to 36% of cases. Those patients, who require insulin to prevent from fetopathy cannot be reliably selected by maternal parameters such as blood glucose and glycosylated hemoglobin values. We recommend the measurement of amniotic fluid insulin between the 28 and 32 weeks of pregnancy to differentiate whether the fetus is compromised or not. Subjects with values above the 97th centile require insulin therapy. Inadequate insulin dosage or delayed fetal hyperinsulinism can be discovered by checking the amniotic fluid insulin level at 33 to 36 weeks. In a total of 88 gestational diabetic patients 19 had raised amniotic fluid insulin levels indicating the onset of diabetic fetopathy at an early stage. Diabetic patients with raised amniotic fluid insulin levels needed large doses of insulin, namely 64.6 +/- 29.5 (Mean +/- SD) U/24 h. This treatment reduced mean blood glucose levels from 98 +/- 9 (Mean +/- SD) mg/dl to 82 +/- 10 mg/dl and was sufficient to prevent from diabetic fetopathy.