Cortisol regulation was investigated in a sample of school-aged maltreated (n = 167) and demographically comparable low-income nonmaltreated (n = 204) boys and girls in the context of a day camp research program. The presence of clinical-level internalizing and clinical-level externalizing symptomatology was determined through adult report and child self report. Children who exhibited clinical-level internalizing problems only, clinical-level externalizing problems only, and comorbid clinical-level internalizing and externalizing problems were identified. Clinical-level cases were more prevalent among the maltreated children. Maltreated children with clinical-level internalizing problems were distinguished by higher morning, afternoon, and average daily cortisol levels across the week of camp attendance. In contrast, nonmaltreated boys with clinical-level externalizing problems emerged as distinct in terms of low levels of morning and average daily levels of cortisol. Maltreated children with comorbid clinical-level internalizing and externalizing problems were more likely not to show the expected diurnal decrease in cortisol. The findings are discussed in terms of the joint impact of maltreatment and different forms of psychopathology on neuroendocrine regulation.