This is a retrospective study to compare the criteria for diagnosis of gestational diabetes mellitus (GDM) by the National Diabetes Data Group (NDDG), and Carpenter and Coustan criteria, and to study the outcome of GDM when diagnosed by the more sensitive criteria. Six hundred and sixty-two pregnant women were included in this study from the medical records between September 1998 and April 2001. GDM was positive in 6.50% of patients according to Carpenter and Coustan and in 4.08% of patients according to NDDG criteria. Women with GDM were older, had higher fasting and glucose challenge test (GCT) glucose levels, and fetal weight than the normal women. Hypoglycemia was observed only in one infant. Regarding pre-term delivery and pre-eclampsia, there was no significant difference between the groups. Age, delivery week and fetal weight of patients who had caesarian delivery were significantly higher than spontaneous vaginal delivery. Prevalence of macrosomia in GDM group was higher than in the normal group. There was a significant correlation between the macrosomia and number of positive blood glucose values during OGTT. In multivariate analyses, fasting, GCT and second hour OGTT blood glucose levels, mean parity, and delivery week were independent risk factors for fetal weight. Carpenter and Coustan criteria is more sensitive than the NDDG criteria and women with GDM had a higher frequency of macrosomia and the frequency of macrosomia increases by the number of positive blood glucose levels during OGTT. Tight glycemic control might decrease the prevalence of caesarian delivery, pre-eclampsia, pre-term delivery and hypoglycemia of the infant.