Self-report questionnaires are in widespread use in the assessment of adults with suspected attention-deficit and hyperactivity disorder (ADHD). Notwithstanding the high degree of validity, these questionnaires are considered to possess, their stand-alone use in assessment for adult ADHD may result in false-positive diagnoses due to the risk of negative response bias. Most of the self-report questionnaires in typical use are based on the diagnostic systems DSM-5 or ICD-10. From a neuropsychological point of view, however, testing of various executive function abilities and attentional performance is important in the assessment of adult ADHD. The present study (N = 211) found no evidence linking executive function (working memory and inhibitory processes) and attentional performance (processing speed) to the results of a self-report questionnaire, the ADHS-LE. The number of failures on the three symptom or performance validity tests (SVT/PVT) used provided the sole, and significant, explanation for the response behavior reported on the ADHS-LE. Of these three SVT/PVTs (the German version of the Structured Inventory of Malingered Symptomatology, SIMS, the reliable digit span, and the standard deviation of simple reaction time), only the SIMS was found to be a significant predictor variable. In the clinical context of this study, 32.6% of subjects produced at least one invalid SVT/PVT result. The use of a more conservative criterion—failure on at least two of the three SVT/PVTs deemed to be feigning ADHD—reduced the proportion of participants generating invalid values to 5%.