OBJECTIVE: To examine whether the association between gestational diabetes mellitus (GDM) and BMI category varies by racial/ethnic group. RESEARCH DESIGN AND METHODS: In a cohort of 123,040 women without recognized pregravid diabetes who delivered babies between 1995 and 2006 at Kaiser Permanente of Northern California, we examined racial/ethnic disparities in the prevalence of GDM by BMI category and the population-attributable risk (PAR) associated with overweight/obesity. RESULTS: Among all racial/ethnic groups, the age-adjusted prevalence of GDM increased with increasing BMI (kg/m(2)) category. However, Asian and Filipina women had a prevalence of GDM of 9.9 and 8.5%, respectively, at a BMI of 22.0-24.9 kg/m(2), whereas in Hispanic, non-Hispanic white, and African American women, the prevalence of GDM was >8.0% at a higher BMI, such as 28-30, 34-36, and >/=37 kg/m(2), respectively. The estimated PARs suggest that the percentage of GDM that could be prevented if all pregnant women were of normal weight (BMI <25.0 kg/m(2)) ranging from 65% for African American women to only 23% among Asian women. CONCLUSIONS: Clinicians should be aware that the BMI thresholds for increased risk of GDM varies by racial/ethnic group and that the risk is high even at relatively low BMI cutoffs in Asian and Filipina women. Asian women may benefit from different prevention strategies in addition to weight management.