Posttraumatic stress disorder (PTSD) is frequently underdiagnosed in maltreated samples. Protective services information is critical for obtaining complete trauma histories and determining whether to survey PTSD symptoms in maltreated children. In the current study, without protective services information to supplement parent and child report, diagnosing PTSD was missed in a significant proportion of the cases. Collaboration between mental health professionals and protective service workers is critical in determining psychiatric diagnoses and treatment needs of children involved with the child welfare system.