A study was undertaken to determine a reference value for fasting plasma glucose in a group of apparently normal pregnant Nigerian women. Three hundred and twenty women were tested; 260 pregnant and 60 non-pregnant. There were 60, 100 and 100 subjects in the first, second and third trimesters, respectively. Fasting plasma glucose was measured in each of the women using the glucose oxidase method. The mean fasting plasma glucose level was 4.64 +/- 0.79 mmol/l in the control group and 3.72 +/- 0.58, 3.78 +/- 0.81 and 3.81 +/- 0.85 mmol/l in the first, second and third trimesters of pregnancy, respectively. Mean fasting plasma glucose+2 standard deviations (SD) of all the pregnant women was 5.3 mmol/l, which is much lower than the World Health Organisation value for the diagnosis of diabetes mellitus.