Objective: To test the validity of the MacArthur Health and Behavior Questionnaire (HBQ) using receiver operating characteristic (ROC) analysis to determine optimal thresholds for the HBQ in predicting Diagnostic Interview Schedule for Children Version-IV (DISC-IV)diagnoses. The roles of child sex, level of impairment, and physical health in understanding of psychopathology were also considered. Method: A sample of 814 8-year-old twin children was recruited from birth records. Mothers were interviewed over the telephone using the HBQ and were also interviewed in person using the DISC-IV. Fathers also completed the HBQ; children completed the parallel module of the Berkeley Puppet Interview. Results: The HBQ identified more cases overall than the DISC-IV. Sex did not contribute to prediction of diagnosis. Impairment, DISC-IV symptom counts, father HBQ, child Berkeley Puppet Interview, and physical health significantly distinguished the HBQ low and high symptom groups. Conclusions: The HBQ is a valid new screening measure of psychopathology for use with children under 9 years of age. It is sensitive to internalizing disorders, which may aid the understanding of depression and anxiety disorders that are often underappreciated in young children.