This study found that early postnatal hypoglycaemia was mainly induced by foetal hyperinsulinaemia, in close relation to maternal hyperglycaemia, even in well-controlled pregnancies of 59 mothers with insulin-treated diabetes mellitus, 29 with insulin-dependent diabetes mellitus and 30 with gestational diabetes mellitus. Ten of the newborn children (17%) had a blood glucose concentration below 1.0 mmol l(-1) at 2 h postnatally. Cord insulin-like growth factor-I or glucagon concentrations were not related to the early decline of blood glucose.