Little is known about the role of hypothalamic-pituitary-adrenal (HPA) axis functioning for children's cognitive development, especially among vulnerable groups. The current study explores the relationship between diurnal cortisol slope and cognitive outcomes among children at the ages of 5 and 6 who have been maltreated as infants and involved with child protective services, using data from the National Survey of Child and Adolescent Well-Being (NSCAW) I (N=158). Multiple regression analyses showed that a greater decline in salivary cortisol from morning to evening was positively associated with scores on applied problems and expressive communication, even after adjustment for confounding. It was also associated with lower odds of cognitive disability. There were null associations with letter-word identification, passage comprehension, auditory comprehension, matrices, and vocabulary. Results suggest that children involved with child protective services as infants, and thus exposed early to likely 'toxic' levels of stressors, may face dysregulation of the HPA axis and particular difficulties in some aspects of cognitive function. Potential explanations and implications for policy are discussed.