We compared the criteria for diagnosis of gestational diabetes mellitus (GDM) of the National Diabetes Data Group (NDDG) and the World Health Organization (WHO) and studied the outcomes of pregnancy. A 50-g glucose screening test and 75-g oral glucose tolerance test (OGTT) were scheduled for 709 pregnant women in the same week between the 24th and 28th week of pregnancy. Blood glucose was measured 1 h after the 50-g glucose screening test and if found to be 7.8 mmol/l or more, a 100-g OGTT was scheduled within 7 days after a 75-g OGTT. The prevalence of GDM was found to be 1.4% (10/709) and 15.7% (111/709) by NDDG and WHO criteria (2 h > or = 7.8 mmol/l), respectively. Using NDDG criteria, all the GDM patients had abnormal 75-g OGTT by WHO criteria. NDDG and WHO criteria were significantly different when compared with normal OGTT by each criteria for age, BMI, pregnancy-induced hypertension, Caesarian delivery, macrosomia and neonatal hypoglycaemia. Of 14 women with macrosomic infants 6 had an abnormal WHO test while only 3 of 14 had an abnormal NDDG test. These findings suggest that WHO criteria GDM patients had significantly worse outcomes of pregnancy and fewer perinatal complications were missed than with the more cumbersome NDDG criteria, and no case of GDM as diagnosed by NDDG criteria was missed.