All newborn children to mothers with gestational diabetes mellitus (GDM) in the county of Orebro were investigated during a one year prospective study. Neonatal macrosomia (birthweight greater than 3 SD) was observed in 27% of children of mothers with GDM and was significantly correlated to the cord C-peptide concentration. Hypoglycaemia (B-glucose less than 1.5 mmol/l) was observed in 38% of the children, most frequently two hours after delivery. Hypoglycaemia was not more common in macrosomic children and could not be predicted by the blood glucose concentration of the mother at delivery or by the cord C-peptide level. It is concluded that mothers with GDM must be intensively treated in order to avoid the occurrence of macrosomia in their infants and that the newborn child must be carefully observed and treated in order to avoid neonatal hypoglycaemia.