Objective - To determine whether the relation between high blood pressure and low birth weight is initiated in utero or during infancy, and whether it changes with age. Design - A longitudinal study of children and three follow up studies of adults. Setting - Farnborough, Preston, and Hertfordshire, England, and a national sample in Britain. Subjects - 1895 children aged 0-10 years, 3240 men and women aged 36 years, 459 men and women aged 46-54 years, and 1231 men and women aged 59-71 years. The birth weight of all subjects had been recorded. Main outcome measures - Systolic blood pressure. Results - At all ages beyond infancy people who had lower birth weight had higher systolic blood pressure. Systolic blood pressure was not related to growth during infancy independently of birth weight. The relation between systolic pressure and birth weight became larger with increasing age so that, after current body mass was allowed for, systolic pressure at ages 64-71 years decreased by 5.2 mm Hg (95% confidence interval 1.8 to 8.6) for every kg increase in birth weight. Conclusions - Essential hypertension is initiated in fetal life. A raised blood pressure is then amplified from infancy to old age, perhaps by a positive feedback mechanism.