Research suggests that early adversity places individuals at risk for psychopathology across the life span. Guided by concepts of allostasis and allostatic load, the present study examined whether early adversity contributes to the development of subsequent internalizing symptoms through its association with traitlike individual differences in hypothalamic-pituitary-adrenal axis regulation. Early adolescent girls (n = 113; M age = 12.30 years) provided saliva samples at waking, 30 min postwaking, and bedtime over 3 days (later assayed for cortisol). Objective contextual stress interviews with adolescents and their mothers were used to assess the accumulation of nine types of early adversity within the family environment. Greater early adversity predicted subsequent increases in internalizing symptoms through lower levels of latent trait cortisol. Traitlike individual differences in hypothalamic-pituitary-adrenal axis activity may be among the mechanisms through which early adversity confers risk for the development of psychopathology.