The reliance on self-reports in detecting noncredible symptom report of attention-deficit/hyperactivity disorder in adulthood (aADHD) has been questioned due to findings showing that symptoms can easily be feigned on self-report scales. In response, Suhr and colleagues developed an infrequency index for the Conners’ Adult ADHD Rating Scale (CII) and provided initial validation for its utility in detecting noncredible symptom report. The aim of this study was to evaluate the utility of the CII in detecting noncredible aADHD symptom report by using a simulation design. Data did not support the validity of the CII for the detection of noncredible aADHD symptoms, as it failed to differentiate instructed malingerers from genuine patients with sufficient accuracy. It is concluded that there is a need for infrequency scales composed of items that were specifically developed to be endorsed infrequently and embedded within valid self-report scales.