Although many studies have examined the relationship of adiposity with neighborhood socioeconomic context in adults, few studies have investigated this relationship during adolescence. Using 10-year annual measurements of body mass index, expressed as z-scores (BMIz), obtained from 775 black and white participants of the National Heart, Lung, and Blood Institute Growth and Health Study, a prospective cohort study of girls from pre- to postadolescence, we used multilevel modeling to investigate whether family socioeconomic status (SES) and neighborhood socioeconomic characteristics (measured by census-tract median family income) explain variation in BMIz trajectory parameters. Analyses controlled for pubertal maturation. We found that lower SES was associated with higher overall levels of BMIz for both white and black girls. Additionally, lower-SES black girls had a more sustained increase in BMIz during early adolescence and reached a higher peak compared to higher-SES black girls and to white girls. Neighborhood income was associated with BMIz trajectory for black girls only. Unexpectedly, among black girls, living in higher-income neighborhoods was associated with higher overall levels of BMIz, controlling for SES. Our findings suggest that neighborhood socioeconomic characteristics may affect adolescent BMIz trajectories differently in different racial/ethnic groups.