Nationally, 15% of children younger than 5 years regularly attend more than 1 child-care arrangement. An association between arrangement multiplicity and children's behavior problems has been identified, but previous research may be susceptible to measurement or omitted variable bias. This study used within-child fixed effects models to examine associations between changes in the number of concurrent, nonparental child-care arrangements and changes in mother- and caregiver-reported behavior among 2- and 3-year-old children in the National Institute for Child Health and Human Development Study of Early Child Care and Youth Development (N = 850). Increases in the number of arrangements were related to increases in children's concurrent behavior problems and decreases in prosocial behaviors, particularly among girls and younger children. Implications for policy and research are discussed.