Data were collected between 1965 and 1971 concerning the physical status and school achievement of a nationally representative sample of Swedish pupils comprised of 360 girls and 380 boys. Pupils were followed from grade 3 through grade 9 of compulsory school. For boys, similar data were collected in connection with military enrollment. Main findings of the survey, concerning variables such as sex differences, physical maturity, mental performance, achievement levels, and social background, are reported. In view of the survey findings, additional studies were planned (1) to investigate more closely the connections between the physical maturity and cognitive levels of pupils in relationship to their social background; (2) to study pupils' self-perceptions in relationship to their physical maturity, cognitive level, and social background; and (3) to investigate the knowledge and views of children and young persons concerning biological development. In pursuit of the first two aims mentioned above, a sample of about 300 pupils in grades 4, 6, and 8 was investigated in two socioeconomically different municipalities outside Stockholm. The third aim was addressed in the course of an international study involving Australia, Britain, North America, and Sweden. Main findings of the international study are reported. (RH)