Prevalence of gestational diabetes was investigated in 693 pregnant patients between the 24th and 28th wk of gestation. A glucose screening test (GST) was performed with a 50-g glucose load, followed by a blood sample 1 h later. Patients with glucose levels >140 mg/dl 1 h after the GST were scheduled for a full oral glucose tolerance test (OGTT). One hundred seven patients had an abnormal GST, and 30 patients (4.3%) were diagnosed as having gestational diabetes mellitus (GDM). The percentage of GDM increased significantly when glucose levels were >180 mg/dl to a maximum of 84.61% when glucose levels were >200 mg/dl. Also, patient age was directly related to GDM, which increased in incidence to 20% when patients >26 yr had an abnormal GST. After delivery, newborn weights were compared between those born to mothers with GDM (n = 30) and those born to mothers with an abnormal GST (n = 77). Patients with an abnormal GST and normal OGTT had 12 (15.58%) macrosomic and 2 premature newborns. However, patients with GDM had 5 (16.66%) macrosomic and no premature newborns. Patients with a normal GST had 7.33% of the macrosomic newborns. There was no perinatal mortality in newborns of GDM mothers; only 1 of the 5 macrosomic newborns presented transient hypoglycemia. Evaluation of 26 GDM patients was possible after delivery, disclosing 3 (11.53%) with non-insulin-dependent diabetes mellitus and 5 (19.23%) with impaired glucose tolerance. These results showed 4.3% undetected GDM in our population and no differences in the proportion of macrosomic newborns between those born to mothers with GDM and those born to mothers with an abnormal GST.