Gestational diabetes (GDM), defined as a carbohydrate intolerance of variable severity with onset of first recognition during pregnancy, is a significantly frequent condition, often pauci- or asymptomatic, which is associated with an increase of maternal, fetal and neonatal morbidity, plus an increased risk of later overt diabetes mellitus in the mother. Good glycemic control significantly reduces the risks of complications, so it is strongly recommended to detect GDM. The screening test suggested by the Workshop-Conference held in Chicago in 1991 and by the NDDG is the 50 g oral glucose load, venous plasma glucose measured 1 h later (OGCT) at the 24th-28th week gestation in all the pregnant women. The test may be administered earlier when factors of metabolic risks are present. If 1h-glycemia is > 140 mg/dl, a full diagnostic test, 100 g oral glucose load, venous plasma glucose fasting and 1, 2, 3 hours later, should be performed. Cut-off values are still controversial. WHO suggest to adopt for diagnosis the same method and criteria used in non pregnant adults: 75 g oral glucose load, venous plasma glucose fasting, 1 and 2 hours later. This load, followed by a 2 hours glycemia, is utilized for screening, too. Nevertheless, WHO detects an interesting status intermediate between GDM and normal tolerance the reduced glycemic tolerance. Outside a formal OGTT, a fasting glycemia > 140 mg/dl and/or a random plasmatic value > 200 mg/dl suggest diabetes.(ABSTRACT TRUNCATED AT 250 WORDS)