We reviewed nine studies in which children's cortisol levels at center daycare were assessed. Our first hypothesis, concerning intraindividual differences in cortisol levels across home and daycare settings, was also tested in a meta-analysis. Our main finding was that at daycare children display higher cortisol levels compared to the home setting. Diurnal patterns revealed significant increases from morning to afternoon, but at daycare only. The combined effect size for seven pertinent studies (n = 303) was r = 0.18 (CI 0.06 - 0.29, p = 0.003). We examined all papers on possible associations between cortisol levels and quality of care, and the influences of age, gender, and children's temperament. Age appeared to be the most significant moderator of this relation. It was shown that the effect of daycare attendance on cortisol excretion was especially notable in children younger than 36 months. We speculate that children in center daycare show elevated cortisol levels because of their stressful interactions in a group setting.