Research on the effect of family structure on childhood obesity is scarce. This study examines the effect of number of parents and number of siblings on US children's body mass index (BMI) and risk of obesity. We conducted a secondary data analysis of the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K), which consists of a nationally representative cohort of children who entered kindergarten in 1998-1999, to examine the effect of family structure on children's body mass index and risk of obesity from kindergarten through 8th grade. Study outcomes were BMI in kindergarten and 8th grade, obesity status in kindergarten and 8th grade, and change in BMI from kindergarten through 8th grade. Multivariate regressions were used to assess the association between family structure and study outcomes while adjusting for other covariates. In 8th grade, children with no siblings had higher BMI (23.7 vs. 22.6; P ≤ 0.01) and higher probability of being obese (25.8 vs. 19.7 %; P ≤ 0.05) than their counterparts with two or more siblings. They also had a larger increase in BMI from kindergarten through 8th grade than children living with two or more siblings (7.3 vs. 6.3; P = 0.02). Our analysis suggests that the association between family structure and obesity persists and even intensifies through 8th grade. These findings have important implications for targeting obesity support and counseling for families.