The General Ability Index (GAI) was compared with Wechsler Adult Intelligence Scale-Third Edition (WAIS-III) Full Scale Intelligence Quotient (FSIQ) from the WAIS-III in data obtained from 381 adults assessed for reported learning or attention problems between 1998 and 2005. Not only did clients with more neurocognitively based disorders (i.e., learning disabilities, attention deficit hyperactivity disorder, and brain injury) demonstrate a larger discrepancy between GAI and FSIQ than did those with mainly psychological conditions (i.e., depression, anxiety, posttraumatic stress disorder), but specific neurocognitive diagnosis also moderated the relationship between these two indices. In neurocognitive disorders, GAI was found to be generally higher than FSIQ, a finding that distinguished this group from a nonclinical sample. There were significant differences in GAI among the clinical groups, and FSIQ was closely related to GAI. Implications for use of the GAI in clinical practice are discussed. (Contains 3 tables, 1 figure, and 1 note.)