The report presents information on intellectual development and school achievement of youths in relation to selected socioeconomic and demographic factors. Estimates were based on data collected by the National Center for Health Statistics from 1966-70. The survey sample consisted of 6,768 adolescents aged 12-17 years. Measurement instruments were the Vocabulary and Block Design subtests of the Wechsler Intelligence Scale for Children (WISC) and the Reading and Arithmetic subtests of the Wide Range Achievement Test (WRAT). Performance on these tests is discussed in terms of parental education, family income, place of residence, progress through school, race, and geographic region. Findings indicated that the educational level of the head of household was most highly correlated with the WISC and WRAT scores and that there was also a strong relationship between family income and test scores. Gross differences in average scores for youths from the two major racial groups were observed, with the average score for white youths being higher than that for black youths on each of the four subtests. Additional research was suggested. Tables, explanation of statistical data and standard errors of estimates are included. (Author/DB)