From 1964-71, 323 pairs of twins and 740 age-matched controls born in 1954-55, were investigated. Compared to schoolchildren in 1880 and 1940 the children in the present study were taller, reached maturity earlier and passed through puberty faster. No differences between socioeconomic strata defined by father's occupation and family income were found either for children's height and weight or for ages at which various criteria of physical maturity were reached. Daughters of manual workers, however, had more weight per height during puberty than did girls of higher status. Children in South Sweden matured half a year later than children in Middle Sweden. The relation between physical and mental development was investigated by comparing the mental performance of pnysically early, average and late maturing girls and boys from grade 3 through 7. Support for analogous spurts in physical and mental growth was found. Social background played a more important role than physical maturity for the children's performance on various achievement and mental ability tests given in grades 3, 5, and 6. Measures of mental performance were the Standard Achievement Test in Swedish and in Mathematics; Differential Ability Test; Intellectual Capacity Test; and grades. (Author/CP)