This study investigated the predictive power of the Wechsler Intelligence Scale for Children-Fourth Edition (WISC-IV) Full Scale IQ (FSIQ), the General Ability Index (GAI), and the WISC-IV index score composites on subsequent reading and math standardized test scores among high-achieving students. The sample consisted of 84 elementary-age students who received an individual cognitive assessment with the WISC-IV in the previous year as part of the application process for gifted and talented programming through their schools. Although there were no significant differences among the mean WISC-IV index scores, 77% of the individual students evidenced statistically significant WISC-IV index score variability. Thus, intraindividual test score variability appears to be the norm among high-achieving students. In spite of this variability, regression analyses indicated that the FSIQ predicted reading comprehension and mathematics achievement better than, or as well as, the GAI or individual scores for verbal comprehension and perceptual reasoning. None of the cognitive variables correlated significantly with achievement scores for Word Reading or Pseudoword Decoding scores, but the FSIQ, GAI, Verbal Comprehension, and Perceptual Reasoning scores predicted reading comprehension. Limitations and directions for future research are discussed.