This investigation explored the classification accuracy of Trail Making Test (TMT; Reitan & Wolfson, 1992) indices for suboptimal effort in a sample of non-litigious acquired brain injury patients seeking outpatient rehabilitation. Patients who exhibited optimal effort completed TMT A and B faster than suboptimal effort patients. Although TMT A time to completion demonstrated adequate sensitivity to suboptimal effort, positive predictive value was fair to poor unless the base rate of suboptimal effort was inflated to 40%. TMT B time to completion yielded poor sensitivity and positive predictive value for suboptimal effort. While TMT A time to completion appears to have some value as a validity indicator, no TMT validity indicator should replace more precise symptom validity tests during neuropsychological assessment.