Screening for gestational diabetes mellitus (GDM) with a 50g glucose challenge has become standard practice in obstetrics. However, there are little data comparing the recommended cut-off points of 130 or 140mg/dl in Asian populations. To determine the optimal threshold for the screening test in an Iranian population, we examined data from 412 pregnant women who underwent both the screening glucose challenge and diagnostic glucose tolerance tests, between the 24th and 28th week's gestation. Receiver operator curve characteristics suggested that 140mg/dl was the optimal threshold for the screening test for GDM in our population. Whilst a 130mg/dl cut-off point increases the sensitivity of the test, the cost implications may be prohibitive in developing countries.