To examine relations between time in nonmaternal care through the first 4.5 years of life and children's socioemotional adjustment, data on social competence and problem behavior were examined when children participating in the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care were 4.5 years of age and when in kindergarten. The more time children spent in any of a variety of nonmaternal care arrangements across the first 4.5 years of life, the more externalizing problems and conflict with adults they manifested at 54 months of age and in kindergarten, as reported by mothers, caregivers, and teachers. These effects remained, for the most part, even when quality, type, and instability of child care were controlled, and when maternal sensitivity and other family background factors were taken into account. The magnitude of quantity of care effects were modest and smaller than those of maternal sensitivity and indicators of family socioeconomic status, though typically greater than those of other features of child care, maternal depression, and infant temperament. There was no apparent threshold for quantity effects. More time in care not only predicted problem behavior measured on a continuous scale in a dose-response pattern but also predicted at-risk (though not clinical) levels of problem behavior, as well as assertiveness, disobedience, and aggression.