STUDY OBJECTIVE--The aim was to examine social and physical correlates of blood pressure in 15 year olds. DESIGN--This was the first, baseline, sweep of a longitudinal survey of 15 year olds based on a two stage stratified clustered random sample. SETTING--The Central Clydeside Conurbation, in the West of Scotland. In 1981 this had a population of 1.7 million and a standardised mortality ratio (relative to Scotland as a whole) of 109. SUBJECTS--A random sample of households containing 15 year olds were approached by Strathclyde Regional Council; 70% agreed to have their names passed on to the MRC (15% refused, 10% could not be contacted, and 5% had moved). Of these 1177, 11% refused to participate, 3% were not contactable/had moved, and 4% did not provide full data. Complete blood pressure data are available for 959 15 year olds (464 males and 495 females). MEASUREMENTS AND MAIN RESULTS--Blood pressure, pulse rate, height, weight, and room temperature were measured by nurses in the subjects' homes. Smoking, drinking, and frequency of vigorous exercise were self reported. Maternal height, birthweight, occupation of head of household, and housing tenure were reported by parents. After controlling for the other variables, systolic blood pressure was significantly associated with weight, pulse rate, and room temperature in males and with weight, pulse rate, housing tenure, smoking, and exercise in females. Diastolic blood pressure was associated with room temperature in males and with mother's height, pulse rate, and housing tenure in females. Controlling for current weight, birthweight was inversely related to systolic blood pressure in males and positively associated in females, though in neither case were these associations statistically significant. CONCLUSIONS--In males, blood pressure was mainly related to anthropometric factors whereas in females it was additionally related to socioeconomic and behavioural variables. Although not reaching significance, the weight standardised relationship between birthweight and systolic blood pressure was consistent for males, but not females, with those reported by recent British studies of children and adults. The longitudinal design of this study will allow us to examine correlates of blood pressure in the same individuals as they reach social and physical maturity.