To study maternal and fetal influences on blood pressure in childhood 405 children aged 4 years who were born and still resident in the Salisbury health district were visited at home for blood pressure and growth measurements. Information on the pregnancy, delivery, and baby was abstracted from the routine obstetric notes. Similar to recent findings in adults, the child's systolic pressure was inversely related to birth weight and positively related to placental weight. Systolic pressure at 4 years increased by 1.2 mm Hg for every SD decrease in the ratio of head circumference to length at birth, and by 1.1 mm Hg for every SD decrease in ponderal index at birth. Mothers whose haemoglobin concentrations fell below 100g/l during pregnancy had children whose systolic pressures were on average 2.9 mm Hg higher than the children of mothers with higher haemoglobin concentrations. Patterns of placental weight, birth weight, head circumference, and length that are associated with high blood pressure in adults are also associated with higher blood pressure in 4 year old children. Identification of the intrauterine influences that lead to these patterns of fetal growth could lead to the primary prevention of hypertension.