Racial/ethnic disparities in adolescent obesity in the U.S. are stark, and the causes of these disparities are largely unknown. We used a cumulative risk index (CRI) to examine the role of social risk in racial/ethnic disparities in obesity. Using the National Longitudinal Study of Adolescent Health and multinomial logistic regression, we examined the role of this CRI in disparities in obesity in the transition from adolescence to adulthood. Controlling for sociodemographic characteristics, compared with White females, Black and Hispanic females had roughly a 50% increase in the odds of newly-developed obesity in adulthood and a 90% increase in the odds of persistent obesity in adolescence and adulthood. After adding our CRI, the Black-White disparities were attenuated to statistical non-significance. Hispanic-White disparities did not change. There were no disparities in obesity for males. Our results suggest that social risk factors accumulate to explain Black-White disparities in obesity for females.