The purpose of this study was to utilize a known-group research design to evaluate the diagnostic utility of the Rarely Missed Index (RMI) of the Wechsler Memory Scale-Third Edition [Wechsler, D. (1997). Wechsler Adult Intelligence Test-3rd Edition. San Antonio, TX: The Psychological Corporation] in assessing response bias in an adult male incarcerated setting. Archival data from a sample of 60 adult male inmates who presented for neuropsychological testing were reviewed. Evaluees were assigned to one of two groups; probable malingerers (PM; n=30) and a group of valid test responders (n=30) (1999). Using the recommended cut-off score of 136 or less, the sensitivity of the RMI was extremely low at 33%. Its specificity was 83%. The positive predictive power of the RMI with the published base rate of 22.8 was 38%; with a negative predictive power of 81%. The positive predictive power of the RMI with a published base rate of 70.5 was 82%. The negative predictive power using a base rate of 70.5% was 34%. Results of the receiver operating characteristics (ROC) analysis indicated that the RMI score with a cut-off 136 or less performed only slightly better than chance in delineating probable malingerers from valid responders in this setting. Overall, the findings suggest that the RMI may not be a reliable index for detecting response bias in this setting and perhaps in similar settings.