Three infants whose diabetic mothers were given chlorpropamide and one infant whose diabetic mother was given acetohexamide up to the time of delivery were studied in the neonatal period because each became severely hypoglycaemic. The sulphonylurea drugs given to the mother crossed the placenta, and fetal plasma concentrations were in the therapeutic range for adults with diabetes mellitus. Each baby had severe hyperinsulinaemia resulting in profound hypoglycaemia. These acutely ill infants needed vigorous and prolonged treatment to correct the hypoglycaemia. In two infants exchange transfusion was performed to remove the drug. These sulphonylurea drugs should not be used to control diabetes mellitus in pregnancy.