This study examined the association between early child care on the development of behavior problems. At the age of 5 years, child care information was collected from parents on a large group of twins who were born between 1985 and 1997. Mothers and fathers rated the behavior of the child at ages 3, 7, and 10 years using the Child Behavior Checklist (CBCL) and teachers reported on the same children's behaviors using the Teacher's Report Form (TRF) at ages 7 and 10. At the age of 3 years, children with nonparental child care experiences had more externalizing problems than children with only parental child care. The long-term effects of quantity of child care were mixed and were only significant for mother ratings and for children from families with a low socioeconomic status. Overall, the effect sizes of child care were very small (effect sizes were between .12 and .23). Children with a larger amount of child care did not show more behavior problems, therefore it was questioned whether the increased levels of behavior problems could be attributed to quantity of child care.