The cohort studied is that of the National Survey of Health and Development, begun in 1946 and still in progress. It is drawn from a population comprising all single, legitimate births to English, Welsh, and Scottish wives of nonmanual and agricultural workers during 1 week and one in four of similar births to wives of manual workers -- a total of 5,362 children. The subjects had been studied every 2 years since birth. Information was collected on growth, illness, social circumstances, and home environment, together with career and employment, marriage, child-rearing techniques when the subjects were fullgrown, and offense data. The most striking and significant associations with later delinquency were the experience of injuries by the males between the ages of 6 and 10, family disruption during the first 5 years of life, and overall loss or prolonged absence of a parent or parents. The apparently 'protective' effect of time spent as the only child or of situations in which parental attention would be optimal, such as in a small family or as the youngest child, was supported. School data corroborated the finding that human relationships with parents and teachers were important. Most studies of delinquency have found strong associations between human relations data and delinquent behavior. None of them, including this longitudinal, prospective study, has found these associations to be amenable to practically useful predictions of outcomes. The specificity of associations with delinquency and of those with illness (i.e., between disturbed family relations and later duodenal ulcers, colitis, and admissions to psychiatric hospitals) shows areas in which further research is required and a cross-disciplinary summary of findings would be of great value. Findings in this and other studies indicate a great need for a better sampling of independent variables to describe the study population's social environment, as an aid to understanding the mechanisms of transmission of the apparent effects of disturbed parent-child relationships. Four notes and 31 references are provided.