BACKGROUND Gestational Diabetes Mellitus (GDM) is frequently associated with adverse pregnancy outcome if not adequately managed. Prompt identification of these women requires a programme of active screening. OBJECTIVE To determine the predictive value of the oral 50-gram glucose challenge test (GCT) in the detection of GDM. METHODS At 24 to 28 weeks of gestation, 134 pregnant women with risk factors for GDM (test group) and another group of 134 consecutive pregnant women without risk factors for GDM (control group) were screened with 50 grams of anhydrous glucose and venous blood obtained one hour later. A diagnostic 3-hour - 100g oral glucose tolerance test (OGTT) was done for all either within one week for those with GCT > or = 130mg/dl (7.2mmol/L) or at 30 to 32 weeks of gestation for those with GCT of less than 130mg/dll RESULTS Of the 134 women in the test group 113 (84.3%) completed the study, as did 109 (81.3%) of the 134 women in the control group. The data from these women were analyzed. Gestational diabetes mellitus was found in 7 (6.2 %) of the test group and 5 (4.6 %) of the control group (p = 0.816), an overall prevalence of 12 (5.4 %) for the study. Among the women with newly diagnosed GDM, 58% had risk factors for glucose intolerance while 42% had no such risk factors. The sensitivity and negative predictive values were all 100% at screening values of 130mg/dl and 140mg/dl; while the specificity was 82.4% at 130mg/dl and 91% at 140mg/dl. This was however not statistically significant (p = 0.619). CONCLUSIONS The plasma glucose value of 140mg/dl in the GCT should be used because of its high sensitivity and higher specificity than does the 130mg/dl screening value. GDM is unlikely to be present if the venous plasma glucose level is less than 140mg/dl, one hour after administration of 50g oral glucose load at 24 to 28 weeks gestational age.