This study investigated the sensitivity and specificity of the most widely used mental health screening instrument in juvenile detention, the Massachusetts Youth Screening Instrument (MAYSI-2), for detecting trauma and symptoms of posttraumatic stress disorder (PTSD) among detained youth. The MAYSI-2 scales measuring Substance Use, Anger/Irritability, Depression/Anxiety, and Traumatic Experiences contributed to the prediction of PTSD symptoms; however, only Depression/Anxiety contributed to the prediction of associated symptoms for girls. The ROC curves technique indicated that the Traumatic Experiences scale was a moderately accurate predictor of likely Full or Partial PTSD but not superior to the other MAYSI-2 scales. These results suggest further work is needed to develop measures that are sensitive to the experiences of delinquent youth with histories of complex trauma.