Much of the research examining the patterns, timing, and socioeconomic characteristics of child overweight has been limited by the lack of longitudinal nationally representative data with sufficiently large or diverse samples. We used the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K), a nationally representative sample of US kindergartners, to identify three distinct patterns of weight gain from kindergarten through eighth grade. The largest group (boys: 59%, girls: 55%) was characterized as having consistently normal weight whereby BMI percentile remained below the 85th percentile. The remaining children (boys: 41%, girls: 45%) fell either into a class characterized as always overweight/at risk of overweight (boys: 27%, girls: 25%) or gradually becoming overweight/at risk for overweight (boys: 15%, girls 20%). We found some evidence that the relationship between socioeconomic status and children's health may operate differently across gender. Among girls, low parental income and education were both significant risk factors for the gradual onset of overweight after beginning Kindergarten. Parental income or changes in parental income were not related to boys' risk of developing overweight after entering Kindergarten; only parents' education. We found that while children of immigrants display higher levels of overweight/at risk for overweight at each grade level, the children of immigrant parents who have had less exposure to the US were more likely to experience early and sustained overweight throughout elementary and middle school, particularly among boys. High rates of overweight as early as kindergarten, combined with race/ethnic differences suggest that interventions should focus on pre-school children's environments.