Prevalence of gestational diabetes was investigated in 222 pregnant women between the 24th and 28th weeks of gestation. A glucose screening test (GST) was performed with a 50 g glucose load, followed by a blood sample 1 hour later. Patients with glycemic levels greater than or equal to 140 mg/dl (7.8 mmol/l) 1 hour after the glucose administration were considered as having a positive GST and scheduled for a full oral glucose tolerance test (OGTT), which was interpreted according to O'Sullivan and Mahan criteria. Fifty-four patients had a positive GST, and 10 of them were diagnosed as having gestational diabetes mellitus. These figures indicate a gestational diabetes prevalence of 4.5% in general population and 18.5% in women selected by GST. The fetal weight index in patients with gestational diabetes was 1.13 +/- 0.10, whereas in normal women this index was significantly lower (p less than 0.01). Patients with positive GST and normal OGTT did not have an increased risk for fetal macrosomia. GST results were significantly higher in pregnant women with advanced age (greater than or equal to 35 years) and with late menarche (greater than or equal to 16 years), but did not vary according to maternal parity. Also, these results were significantly related with the number of risk factors for gestational diabetes encountered in a given patient, but not with any of them as examined alone, except for age. We conclude that GST performed in the 24th-28th weeks of gestation reveals 4.5% undetected gestational diabetes in our population. Certain maternal features (age, menarche, number of risk factors) influence on GST results.