In order to determine the prevalence of glucose intolerance in pregnancy, 2,230 consecutive women attending the antenatal clinic at the Aga Khan University Medical Centre in Karachi, Pakistan were subjected on the first antenatal visit, irrespective of gestational age, to a 75 g glucose challenge followed 2 hr later by plasma glucose determination. The test, was repeated at 28-32 weeks of gestation for those patients who had an abnormal initial screen at less than 28 weeks gestation followed by a normal glucose tolerance test and for those who had a risk factor for gestational diabetes even though the initial screen at less than 28 weeks gestation was normal. The initial glucose challenge test was abnormal (2 hr plasma glucose greater than 140 mg%) in 8.6% of the screened population. An oral glucose tolerance test on these patients revealed a prevalence for the entire population of 3.5% of gestational diabetes and 1.9% of impaired glucose tolerance test based on the modified O'Sullivan criteria. Patients with abnormal glucose tolerance test were older, had higher parity, a past history of macrosomia and a family history of diabetes compared to the controls. These patients also had a higher incidence of preterm labour and caesarean section. In the neonates hypoglycemia and hyperbilirubinemia were similarly higher. The fetal abnormality rate was 5.6% and the perinatal mortality was 28/1,000 which were higher than the controls.