Previous research fails to find a consistent association between obesity and acculturation for children. We theorize that social isolation shelters children of immigrants from the U.S. "obesiogenic" environment, but this protective effect is offset by immigrant parents' limited capacity to identify and manage this health risk in the United States. We further theorize that these factors affect boys more than girls. We use data from over 20,000 children in the Early Childhood Longitudinal Study Kindergarten Cohort and find that boys whose parents were raised outside the United States weighed more and gained weight faster than any other group. However, within this group, sons of low English-proficient parents gained weight more slowly than sons of English-proficient parents. The results thus suggest that two dimensions of low acculturation--foreign place of socialization and social isolation--affect children's weight gain in opposite directions and are more important for boys than girls. (Contains 3 tables, 4 figures and 1 note.)