OBJECTIVE: To compare the prevalence of large-for-gestational-age (LGA) newborns across categories of body mass index (BMI) in five racial and ethnic groups. METHODS: This cohort study examined 7,468 women with gestational diabetes mellitus (GDM) who delivered a live newborn between 1995 and 2006 at Kaiser Permanente Northern California. The racial and ethnic groups were non-Hispanic white, African American, Hispanic, Asian, and Filipina. The BMI was classified using the World Health Organization International guidelines (normal, 18.50-24.99; overweight, 25.00-29.99; obese, 30.00-34.99; obese class II, 35.00 or higher). Having an LGA newborn was defined as birth weight more than 90th percentile for the study population’s race or ethnicity and gestational age–specific birth weight distribution. Logistic regression was used to estimate odds of having an LGA newborn by BMI and race and ethnicity. RESULTS: Overall prevalence of LGA newborns was highest in African American women (25.1%), lowest in Asians (13.9%), and intermediate among Hispanic (17.3%), white (16.4%), and Filipina women (15.3%). The highest increased risk of LGA newborns was observed among women with class II obesity in most racial and ethnic groups, and African American and Asian women with class II obesity had a four-fold increased risk of LGA newborns compared with women of normal weight in the same racial and ethnic group. CONCLUSIONS: African American women with GDM have a greater risk of LGA newborns at a lower BMI than other racial and ethnic groups. Clinicians should be aware that among women with GDM, there may be significant racial and ethnic differences in the risk of LGA newborns by BMI threshold.