Cortisol levels were compared in children born preterm at extremely low gestational age (ELGA; 24-28 weeks), very low gestational age (VGLA; 29-32 weeks), and full-term in response to cognitive assessment at 18 months corrected age (CA). Further, we investigated the relationship between maternal interactive behaviors and child internalizing behaviors (rated by the mother) in relation to child cortisol levels. EGLA children had higher "pretest" cortisol levels and a different pattern of cortisol response to cognitive assessment compared to VGLA and full-terms. Higher cortisol levels in ELGA, but not full-term, children were associated with less optimal mother interactive behavior. Moreover, the pattern of cortisol change was related to internalizing behaviors among ELGA, and to a lesser degree VLGA children. In conclusion, our findings suggest altered programming of the hypothalamic-pituitary-adrenal (HPA) axis in preterm children, as well as their greater sensitivity to environmental context such as maternal interactive behavior.