Previous work has shown that full-day center-based child care is associated with increased physiologic stress for many young children (e.g., Tout, de Haan, Campbell, & Gunnar, 1998; Watamura, Sebanc, & Gunnar, 2002). Specifically, increasing cortisol from morning to afternoon at full-day child care in contrast to decreasing cortisol across the day for these same children at home has been repeatedly demonstrated for toddlers and preschoolers. Factors that have been related to rising cortisol across the day at child care include the child's age (rising cortisol at child care between 2 and 5 years, but not for infants or older children, Dettling, Parker, Lane, Sebanc, & Gunnar, 2000; Watamura, Donzella, Alwin, & Gunnar, 2003 ), and global classroom quality (higher quality classrooms having fewer children showing rising cortisol across the day, e.g., Sims, Guilfoyle, & Parry, 2006). Some studies have also identified relations with particular temperaments (e.g., Watamura, et al., 2002). Furthermore, recent work suggests that rising cortisol at child care may be associated with health risk in the form of lower antibody levels (Watamura, Coe, Laudenslager, & Robertson, 2009) and that early child care may be associated with attenuated cortisol in adolescence (Roisman, et al., 2009). This study extended the previous work on stress reactivity at child care by addressing: 1) whether children's attachment to their primary caregiver was associated with how they respond to the challenge of child care; 2) whether negative child temperament alone or in combination with insecure attachment was associated with how they respond to the challenge of child care; and 3) whether the nature of children's attachment to their teacher explained how they respond to the challenge of child care. Saliva samples were collected on three child care days at mid-morning and mid-afternoon. Temperament was assessed by both parents (CBQ; Putnam & Rothbart, 2006) and teachers (T-CBQ; Gunnar, Tout, de Haan, Pierce, S., & Stanbury, 1997) and attachment security was measured using the AQS (Waters, 1995) with the primary caregiver and lead teacher. Children rated as high in both security and dependency to their primary caregiver were more likely than secure children with low dependency or insecure children to show rising cortisol across the day at child care. In addition, children who scored lower on the teacher sorted AQS sociability factor (but not on the CBQ or T-CBQ temperament factors) were more likely to show rising cortisol across the day at child care. Finally, higher security with teachers was associated with falling cortisol across the day at child care. These results suggest that in the process of acquiring a solidified working model of secure attachment to primary caregivers, children who are secure but still dependent may be more stress reactive to out-of-home care. Furthermore, as these results demonstrate that secure relationships with teachers may buffer children from flat or rising cortisol at child care, they suggest a concrete avenue for intervention. (author abstract)