Advancements in medicine have increased survival of Fontan patients with over 85% expected to survive into adulthood. There has been an increase in the prevalence of cardiovascular risk factors among adults with congenital heart disease. Diabetes is a risk factor for poor prognosis in adults with cardiovascular disease; yet little is known about diabetes in adolescents and young adults (AYA) with Fontan circulation. Given that 1 in 5 adolescents and 1 in 4 young adults in the U.S. have prediabetes, we routinely screen for diabetes. We report results of HgA1c screening in our AYA with Fontan circulation. This is a single center retrospective study of Fontan patients seen in a pediatric multi-disciplinary single ventricle clinic. As part of standard surveillance care, all patients greater than 10 years of age undergo screening for diabetes with HgA1c. Abnormal HbA1c was defined as HgA1c >5.7% (pre-diabetes). Weight was defined by body mass index (BMI) percentiles with underweight being < 5th, overweight between 85 to 95th and obesity being >95th percentile. Family history of diabetes mellitus in first-degree relatives was collected from chart review. Data was presented as median (25th, 75th percentiles), and comparisons between groups were performed using chi-squared tests. Between 2015 and 2021, 66 patients >10 years of age seen in our Fontan clinic underwent HgA1c screening. Median age was 14.8 (12.6, 16.6) years old, 62% were male, and race included 44% non-Hispanic White, 41% Hispanic, 6% Asian, 5% African American, and 4% other. Median BMI was 49.6 (26.1, 87.7)th percentile, with 13% obese, 16% overweight, 65% normo-weight and 6% underweight. Median HgA1c was elevated at 5.7 (5.5, 5.9), with 50% of the cohort (n=33) with HgA1c >5.7% and 2% (n=1) with HgA1c >6.5% (Figure 1A). There was no correlation between BMI and HgA1c (Figure 1B), with no difference in the prevalence of overweight or obesity between those with and without abnormal HgA1c (31% versus 27%, p=0.69). While 20% had a family history of diabetes, there was no difference in family history between those with and without abnormal HgA1c (21% versus 19%, p=0.85). There was no difference in prevalence of abnormal HgA1c by race (43% for non-Hispanic White, 50% for Hispanic, 75% for Asian, and 67% for African American, p=0.56). Additionally, there was no difference in time from Fontan, hemoglobin, and other serum markers of liver, renal, and lipid disease between those with and without abnormal HgA1c.