OBJECTIVE To compare the diagnostic performances of 75g and 100g oral glucose tolerance tests in detecting Gestational Diabetes Mellitus in Nigerian pregnant women. METHODS 248 women in 3rd trimester attending antenatal clinic of the Lagos University Teaching Hospital, Lagos, between November 1997 and July 1999 were randomly subjected to standard oral glucose tolerance tests (OGTT). 110 had 100g OGTT while 138 had 75g OGTT. The plasma glucose response (PGR) was assessed and glucose tolerance status of each patient was determined using WHO (1985) criteria to interpret 75g OGTT and National Diabetes Data Group (NDDG) (1979) criteria for 100g OGTT. The PGR in the two study groups were compared. The prevalence rates of GDM using either of the two criteria were evaluated and compared. Incidences of foetal macrosomia in GDM cases diagnosed by either set of criteria were also compared. RESULTS The mean age of the study subjects was 30.7(+/-4.2) years while the BMI was 25.4(+/-4.9) kg/m2. The mean parity was 1.33. Traditional risk factors for GDM were found in 47.5% of them. The plasma glucose response (PGR) to 100g OGTT was found to be higher than that of 75g OGTT at 1 hour, 2hour and 3 hour sampling times but the difference was only significant at 3rd hour (p values = 0.68, 0.137, 0.007 respectively). The total area under the glucose response curve (AUC) for 75g OGTT was 345.1 (+/-49.5) AAU while for 100g OGTT, it was 363.4(+/-61.4) AAU. The difference was not statistically significant (p value >0.05). The prevalence rate of GDM diagnosed by 75g OGTT was 11.6% while that of 100g OGTT was 4.5 %. The difference was significant (p value = 0.04). The incidence rate of foetal macrosomia among GDM cases diagnosed by 100g OGTT was 66.7% as against 23.1% among those diagnosed by 75g OGTT. Statistical difference could not be determined because of the small number. CONCLUSIONS Plasma glucose response to OGTT among Nigerian pregnant women has little or no respect for the load of the glucose administered. 100g OGTT- based NDDG criteria was more stringent than 75g OGTT-based WHO criteria in identifying GDM. However it appears to be more specific for detecting the complications associated with the condition though it will require a larger study to validate this claim.