The aim of this study was to determine the effect of a high carbohydrate preparatory diet on the performance of the 3-hour oral glucose tolerance test (GTT) in pregnancy. This prospective clinical trial was performed at a university hospital outpatient obstetric clinic. Gravidas who had an abnormal blood sugar screening test were asked to ingest one of two preparatory diets: > or = 150g/day of carbohydrate for 3 days (Carbo) or six Snickers candy bars per day for 3 days (Candy), or to remain on their usual diet (Ad lib) prior to the GTT. A 100g oral GTT was performed after an overnight fast, and blood glucose values were determined at 1, 2, and 3 hours. Patients with either a fasting whole blood glucose > 120 mg/dl or > or = two abnormal values on the GTT were considered to have gestational diabetes mellitus and received diet therapy. Women who failed diet therapy also received insulin. Our primary outcome parameter was the intergroup incidence of gestational diabetes mellitus. Other parameters included the mean 1-, 2-, and 3-hour glucose values and the rates of both cesarean delivery and large-for-gestational age infants. Of the 354 women studied, 108 entered the Carbo group, 105 entered the Candy group, and 141 comprised the Ad Lib group. The three groups were comparable with regard to historic and demographic risk factors for gestational diabetes mellitus. After the GTT, 29% of the Carbo group were considered to have gestational diabetes mellitus vs. 28% in both the Ad lib and Candy groups (P = 0.98). Additionally, both the mean intergroup fasting, 1-hour, 2-hour, and 3-hour whole blood glucose values and selected clinical outcomes were similar (P = 0.35-0.99). Compared to usual dietary intake, recommending a high carbohydrate diet had a negligible effect on the 100g oral GTT in pregnancy.