The current study examines the extent to which associations between internalizing problems, body mass index (BMI), and language skills from early (36 months) to late childhood (fifth grade) are due to relatively stable between-child differences, time-specific correlations, or cross-lagged paths. Data from the NICHD study, Early Child Care and Youth Development (N = 1,364) were used. Results showed that internalizing problems and language are significantly and negatively correlated due to relatively stable between-child differences, with some evidence of positive cross-lagged paths, where better language at 36 months, 54 months, and third grade predicted more internalizing problems at the subsequent timepoint, and more internalizing problems at third grade predicted better language at fifth grade. Time-specific associations for BMI showed a negative correlation with language at 36 months and a positive correlation with internalizing problems at 54 months only. Additionally, higher internalizing at third grade predicted higher BMI at fifth grade, though the association was small and no other cross-lagged paths between internalizing and BMI emerged. These findings suggest that previous research documenting cross-lagged associations between BMI and internalizing problems, and between language and internalizing problems may be biased due to between child differences not fully controlled for in prior models. Implications for understanding these key aspects of youth's healthy development are considered. (PsycInfo Database Record (c) 2022 APA, all rights reserved).