In a prospective controlled trial, we studied the effect of tight metabolic control on the outcomes of 102 gestational diabetes mellitus (GDM) pregnancies compared with outcomes of 102 matched nondiabetic control pregnancies. Women with GDM were treated to achieve and maintain a blood glucose concentration of P < .05) increase of macrosomia and umbilical artery acidosis compared with the well-treated GDM group. The study reported herein demonstrates that excess mortality and morbidity typically observed in GDM can be prevented by early institution of tight metabolic control, which required insulin in 86% of our patients.