Research Findings: Data from a national sample of Portuguese preschool centers were used to examine the relationship between age of start and number of hours in child care and levels of externalizing and prosocial behaviors with peers. Participants were both parents and teachers of 543 children (mean age = 4.5 years, 50.6% girls). Children started child care between 3 and 64 months and were attending child care 1-9 hr per day. The child care centers' classrooms had adult-child ratios between 5 and 8.7 and group sizes between 15 and 26 children. Externalizing and prosocial behavior with peers was assessed with the Social Competence and Behavior Evaluation Scale-Short Form completed by the 3 adult informants. Control variables included family sociodemographics and education level, maternal parenting style assessed with the Parenting Styles and Dimensions Questionnaire, and maternal stress assessed with the Parenting Experiences questionnaire. Practice or Policy: Both the number of hours per week in child care and an earlier start of center-based child care had modest but significant associations with dimensional scores from teachers' reports of externalizing behavior but not with mothers' or fathers' reports, suggesting that externalizing behavior with peers could be regarded as context specific to peer relationships in group child care. There was no evidence that the quantity of exposure to child care per se could be a substantial risk factor for severe levels of externalizing behavior. Prosocial behavior with peers was not significantly associated with the number of hours in child care or with the age of entry into group child care.