This study employed observed factor index scores as well as latent ability constructs from the "Wechsler Intelligence Scale for Children-Fourth Edition" (WISC-IV; Wechsler, 2003) in estimating reading and mathematics achievement on the "Wechsler Individual Achievement Test-Second Edition" (WIAT-II; Wechsler, 2002). Participants were the nationally stratified linking sample (N = 498) of the WISC-IV and WIAT-II. Observed scores from the WISC-IV were analyzed using hierarchical multiple regression analysis. Although the factor index scores provided a statistically significant increment over the Full Scale IQ, the size of the improvement was too small to be of clinical utility. Observed WISC-IV subtest scores were also subjected to structural equation modeling (SEM) analyses. Subtest scores from the WISC-IV were fit to a general factor ("g") and four ability constructs corresponding to factor indexes from the WISC-IV (Verbal Comprehension, Perceptual Reasoning, Working Memory, and Processing Speed). For both reading and mathematics, only "g" (0.55 and 0.77, respectively) and Verbal Comprehension (0.37 and 0.17, respectively) were significant influences. Thus, when using observed scores to predict reading and mathematics achievement, it may only be necessary to consider the Full Scale IQ. In contrast, both "g" and Verbal Comprehension may be required for explanatory research.