Neonatal morbidity was assessed in the offspring of 878 mothers with gestational diabetes mellitus (GDM), 132 mothers with pre-GDM, and 380 control subjects. Compared with the control group, the GDM group had a higher incidence of complications, including macrosomia, hypoglycemia, hyperbilirubinemia, hypocalcemia, polycythemia, and major congenital anomalies (P less than 0.05). Although our GDM patients were stringently managed with diet or diet plus insulin, as indicated, and maintained almost euglycemic values, these neonatal complications could not be eliminated. Our data may be consistent with observations published during the last decade that even subtle degrees of maternal hyperglycemia can have a detrimental effect on perinatal outcome. Most neonatal complications readily respond to therapy if diagnosed and treated early and promptly. Macrosomia can have a detrimental effect on delivery (trauma) and later long-term implications during childhood. Tight metabolic control with diet and, when indicated, insulin treatment may be advantageous in reducing fetal birth weight. Criteria of how tight the metabolic control should be remain to be accurately defined.