Young children in institutional care experience conditions that are incompatible with their needs for attachment relationships. As a result, early institutionalization is expected to have lasting effects on the regulation of the hypothalamic–pituitary–adrenocortical (HPA) axis. The current study tested whether early institutionalization has persistent consequences for diurnal HPA axis outcomes among 130 children who had been adopted internationally between the ages of 6 and 48 months. Daily cortisol samples were collected from children at two time points: shortly after adoption (average of 5.3 months after adoption) and approximately 3 years later (average of 39.2 months after adoption). Shortly after adoption, children who had experienced a long duration of institutional care had lower morning cortisol levels and more blunted declines in cortisol across the day than children who experienced minimal or no institutional care. Three years later, children who had experienced a long duration of institutionalization continued to exhibit low morning cortisol levels and also exhibited low bedtime cortisol levels. Altogether, these results support the idea that early adversity results in the downregulation of the HPA axis and suggest that the effects of institutionalization on HPA axis functioning may persist several years after children are adopted into highly enriched families.