An intensive analysis of the relative value of using the Vocabulary and Block Design subtests of the Wechsler Intelligence Scale for Children (WISC) to predict Full Scale IQ based on six verbal and five performance WISC subtests was conducted. The tests were administered to 1,310 6 to 11 year old Anglo, Negro, and Mexican-American school children. Separate analyses were performed by sex, age, and socioeconomic level for each of the ethnic groups. The findings support the results from other studies, that is, that Mexican-American children do relatively better on the performance than on the verbal tests. It is noted that the size of the standard error of estimated Full Scale IQ for the Vocabulary and Block Design subtests could lead to misclassifications of individuals in predicting IQ scores of less than 85, 80, and 70 for diagnostic purposes. However, it is suggested that both scores could be used to provide estimated IQ scores for groups of individuals. It is concluded that no other combination of two WISC subtests would have produced better overall predictions for all three ethnic groups. The sum of the scaled scores for the Vocabulary and Block Design subtests correlated .880 with Full Scale IQ. (Author)