To assess the potential value of glycosylated hemoglobin measurement (HbA1) in screening for gestational diabetes mellitus (GDM), HbA1 by a colorimetric method, plasma glucose level 1 hr after 50 g oral glucose loading, and 3-hr 100 g oral glucose tolerance test (3-hr OGTT) were performed in 334 pregnant women at Ramathibodi Hospital. These subjects carried high risk factors of GDM. Gestational ages varied from 24 to 38 wks. Twenty-four cases were diabetic (7.2%) by O'Sullivan and Mahan's criteria (1964). As a screening test to select subjects for 3-hr OGTT, plasma glucose level 1 hr after 50 g oral glucose load (plasma glucose level 7.8 mmol/L and above) has sensitivity: 87.50 per cent, specificity: 64.19 per cent. HbA1 level of 5.60 per cent (upper 95% confidence limit of the mean) and above yield sensitivity: 66.67 per cent, specificity: 61.29 per cent. Thus, glycosylated hemoglobin measurement as a screening test for GDM is not as effective as the conventional 50 g oral glucose loading test.