The WISC-R factors, Verbal Comprehension, Perceptual Organization and Freedom from Distractibility (FFD) were examined for their discriminant validity in diagnosing children with Attention Deficit Hyperactivity Disorder (ADHD), ADHD with co-occurring Conduct Disorder (ADHD + CD) and those comprising a clinic control population. Contrary to expectations and clinical practice, the FFD factor did not significantly discriminate among children comprising the three diagnostic groups. The verbal comprehension and perceptual organizational factors significantly distinguished between children with and without conduct disorder, but not between the clinic control and ADHD children. Therefore, the ADHD + CD group's performance on the WISC-R factors qualitatively differs from children with internalizing disorders or ADHD, and as such is most likely a separate population. Furthermore, the use of the FFD factor did not provide sufficient sensitivity for a discriminative diagnosis (a miss rate of 50 percent for clinic control, 64.3 percent for ADHD and 38.1 percent for ADHD + CD).