Prior studies have shown that the Youth Psychopathic Traits Inventory (YPI) holds promise as a self-report tool for assessing psychopathic traits in detained adolescents. However, these studies have been conducted in a research context where anonymity and confidentiality are provided. Few studies have examined the usefulness of the YPI in clinical settings. To address this research gap, the present study examined data from 1,559 detained boys who completed the YPI as part of a clinical protocol. Official criminal records were available for a subsample (n = 848), allowing us to test the prognostic usefulness of the YPI. Results of confirmatory factor analyses, overall, support the proposed 3-factor structure, though model fit indices were not as good in Dutch boys compared to boys from other ethnic groups. Measurement invariance tests showed that the YPI scores are manifested in the same way across all 4 ethnic groups and suggest that means scores between the 4 ethnic groups are comparable. The YPI scores were internally consistent, and correlations with external variables, including aggression and conduct problems, support the convergent validity of the interpretation of YPI scores. Finally, results demonstrated that YPI scores were not significantly positively related to future criminality. In conclusion, this study suggests that the YPI may hold promise as a self-report tool for assessing psychopathic traits in detained male adolescents during a clinical protocol. However, the finding that the YPI did not predict future offending suggests that this tool should not yet be used for risk assessment purposes in forensic settings.