Background: Callous-unemotional (CU) traits have been found to index an important subgroup of antisocial youth who are at high risk for developing psychopathic personality pathology, and for becoming severe and persistent offenders. On the basis of such research findings, the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, have included a "with limited prosocial emotions" specifier in the diagnostic criteria for conduct disorder to designate a subtype with high levels of CU traits. This creates the need for psychometrically sound measures for the assessment of these traits. The self-report questionnaire Inventory of Callous-Unemotional Traits (ICU) was designed to provide an efficient, reliable, and valid measure of CU traits among youth populations. Method: Eighty Danish adolescent boys between the ages of 15 to 18 years in secure institutions were assessed concurrently with the ICU, the Psychopathy Checklist: Youth Version (PCL:YV), self-report measures of aggression and empathy, and ratings of psychosocial problems. Approximately nine days later, the ICU was readministered in a subset of the sample (n = 40) to examine test-retest reliability. Results: Internal consistency was satisfactory, and test-retest reliability was excellent. Concurrent validity associations with the PCL:YV ranged from moderate to high. The ICU displayed excellent discriminative validity for identifying persons who displayed high levels of psychopathic traits. CU traits were also found to be associated with psychosocial impairments, aggression, and reduced empathy. Conclusions: Overall, these findings support the reliability; construct validity, and criterion validity of the ICU.