Diagnosis of adult attention-deficit/hyperactivity disorder (ADHD) adults is difficult, as neither symptom report nor neuropsychological findings are specific to ADHD. Few studies address the possibility that noncredible performance influences both symptom report and neuropsychological findings. The present study utilized archival data from young adults referred for concerns about ADHD, divided into three groups: (1) those who failed a measure of noncredible performance (the Word Memory Test; WMT), (2) those who met diagnostic criteria for ADHD, and (3) controls with psychological symptoms but no ADHD. Results showed a 31% failure rate on the WMT. Those who failed the WMT showed clinical levels of self-reported ADHD symptoms and impaired neuropsychological performance. Neither self-report measures nor neuropsychological tests could distinguish ADHD from psychological controls, with the exception of self-reported current hyperactive/impulsive symptoms and Stroop interference. Results underscore the effect of noncredible performance on both self-report and cognitive measures in ADHD.