Performance on the Medical Symptom Validity Test (MSVT) was examined in 193 consecutively referred patients aged 8 through 17 years who had sustained a mild traumatic brain injury. A total of 33 participants failed to meet actuarial criteria for valid effort on the MSVT. After accounting for possible false positives and false negatives, the base rate of suboptimal effort in this clinical sample was 17%. Only one MSVT failure was thought to be influenced by litigation. The present results suggest that a sizable minority of children is capable of putting forth suboptimal effort during neuropsychological exam, even when external incentives are not readily apparent. The MSVT appears to have good potential value as an objective measure for detecting symptom invalidity in school-age youth.