This paper focuses on continuity and change in cognitive ability profile, as represented by the pattern of subtest scores obtained on the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) and Wechsler Intelligence Scale for Children (WISC) during middle childhood. A large number of twins have been tested annually in a longitudinal study, and their scores were evaluated for age-to-age consistency and for withinpair concordance. The following questions were addressed: (1) Does each child display a consistent pattern of subtest scores from age to age? (2) How can genuine transformations in the cognitive profile be separated from measurement error? (3) Do twins match one another for cognitive profile and for systematic changes over ages? The results showed that Verbal IQ scores and Performance IQ scores were more reliable than subtest scores, and the systematic effects of age-to-age consistency and twin concordance were more strongly represented in the V-IQ and P-IQ scores. Within the limits of subtest reliability, however, the cognitive profile was consistently reproduced over ages and was displayed in common by monozygotic (MZ) twins. Some systematic changes in the cognitive profile were detected over longer intervals from childhood to adolescence. Genetic influences were apparent for both continuity and change, although continuity played the more prominent role. The results also confirmed that a high test-retest reliability was a prime requisite for extracting the systematic variance attributable to continuity and change in cognitive ability profile.