To determine whether the 50 gm, 1-hour plasma glucose screening test for gestational diabetes should be administered in the fasted or fed state, 50 presumed normal and 20 gestational diabetic pregnant women in the early third trimester underwent this test twice, once under each condition, within a 1-week interval. There was no difference in test results under the two conditions among the normal individuals (fasted 118.4 +/- 24.7 mg/dl; fed 115.8 +/- 23.4 mg/dl). However, when the test was administered to women with known gestational diabetes, the result was significantly (p = 0.011) higher if patients were fasted (173.9 +/- 28.8 mg/dl) than if they had been given a standard 600 kcal meal 1 hour previously (154.8 +/- 24.1 mg/dl). The effect of these two conditions on the sensitivity and specificity of the screening test is described, and it is suggested that the threshold for glucose tolerance testing be 130 mg/dl if the test is administered in the fed state.