OBJECTIVE:To study the concurrent validity and the inter-rater reliability of the Post-Concussion Symptoms Questionnaire. DESIGN:The approach was to study the concurrent validity of the Post-Concussion Symptoms Questionnaire when used as an interview questionnaire compared with a self-report questionnaire administered by the patients. The inter-rater reliability was also studied when 2 different raters administered the Post-Concussion Symptoms Questionnaire interview. PATIENTS: Thirty-five patients with mild traumatic brain injury were consecutively contacted by telephone and asked whether they would be willing to participate in a follow-up intervention. METHODS: The Post-Concussion Symptoms Questionnaire was completed by the patients, who answered "Yes" or "No" to the standardized questions. The patients were then interviewed to check the certain "Yes" or "No" answers, 0-10 days after having completed the first Post-Concussion Symptoms Questionnaire. The raters filled in their ratings independently. RESULTS: The concurrent validity of answers in the questionnaire compared with those in the interview ranged from 82% to 100% agreement. The inter-rater reliability results ranged from 93% to 100% agreement between the raters. CONCLUSION: The Post-Concussion Symptoms Questionnaire with answers of "Yes" or "No" is a valid instrument. High reliability was found between the raters.