Objective To determine whether glucose intolerance can be identified early in gestation in a high-risk population so that early intervention can be planned to prevent associated morbidity. Research Design and Methods After appropriate dietary preparation, patients with a high risk for gestational diabetes underwent a 50-g oral glucose screening test during fasting. Patients were tested on enrollment and every 10 wk until delivery. Those with a 1-h plasma glucose value of ≥7.5 mM underwent a 100-g oral glucose tolerance test. Gestational diabetes was based on either a markedly abnormal 50-g screening test or abnormal 100-g oral glucose tolerance test. Results Ten of 15 (66%) patients who developed gestational diabetes were diagnosed during the first half of the pregnancy. Six were diagnosed in the first trimester. If the definition of an abnormal 1-h plasma glucose value was lowered from 7.5 to 7.2 mM, an additional 2 patients could have been identified in the first trimester with an improvement in sensitivity from 70 to 91% with only a slight drop in specificity (from 91 to 88%). Diagnosis of gestational diabetes was not enhanced by measuring plasma insulin concentrations or insulin-glucose molar ratios. Conclusions The diagnosis of gestational diabetes in a high-risk population can be made in the first half of pregnancy. Early diagnosis should permit evaluation of intervention strategies, which may result in improved perinatal outcome.