Currently screening the antenatal population for gestational diabetes mellitus (GDM) is carried out by the administration of the 75 gram glucose challenge test (GCT) in the 3rd trimester. The GCT can also be given in early pregnancy and the benefits of this screening strategy are assessed in this study. In early pregnancy, a GCT > or = 6.5mmol/L is considered a positive result for which a glucose tolerance test (GTT) should be performed at 28 weeks' gestation, however, a GCT > or = 10.0mol/L is an indication for an immediate GTT in order to detect those patients with a severe disorder of carbohydrate metabolism which predated the pregnancy. The study consists of 1500 antenatal patients in whom a GCT was given in early pregnancy and subsequently a GTT was performed. The diagnosis of GDM was made in 164 patients (10.9%). Statistical analysis of the results indicate a positive GCT to have a sensitivity of 79% and a specificity of 61% in early pregnancy which is comparable to the GCT given in the 3rd trimester. The early administration of the GCT is considered an effective method of screening for GDM and has the additional benefit of detecting patients with pre-existing abnormalities of carbohydrate metabolism.