Factors operating in fetal life or during childhood may be important in determining fibrinogen and factor VII concentrations in adult life, and particularly in explaining social gradients in cardiovascular disease risk. In 1994, the authors measured fibrinogen and factor VIIc levels in 641 children aged 10-11 years (61% response rate) from schools in five towns in England and Wales. Birth weight was obtained by maternal recall, and other data on measures of fetal growth were obtained from birth records. Fibrinogen levels were higher in girls (258.8 mg/dl) than in boys (245.4 mg/dl) (95% confidence interval (CI) for difference: 5.5, 21.5). Fibrinogen and factor VIIc levels were linearly related to adiposity, rising by 37.1 mg/dl (95% CI: 24.7, 49.5) and 13.0% of standard (95% CI: 6.3, 19.7), respectively, between the bottom and top quintiles of ponderal index (weight (kg)/height (m)3). Fibrinogen was independently related to heart rate (p < 0.001) and was negatively but nonsignificantly related to measures of physical activity. Factor VIIc was positively correlated with total cholesterol (p < 0.001). No relations were found with measures of fetal growth or social class. These data do not support the concept that fibrinogen or factor VII levels are determined in utero or by social factors in childhood. Adiposity and physical training appear to be the important determinants of fibrinogen and factor VII levels in childhood.