It has been suggested that essential hypertension is determined by 2 separate mechanisms: a growth-promoting process in childhood and a self-perpetuating mechanism in adult life. We report a clinical study of 500 people taken from a cohort of 7086 men and women who were born in Helsinki from 1924 to 1933, and whose body size at birth was recorded. As expected, blood pressure levels were inversely related to birthweight and birth length. These associations, however, were confined to the 213 people who had previously been diagnosed as having hypertension. In them, a 1-kg increase in birthweight was associated with a 6.4-mm Hg (95% confidence interval, 1.0 to 11.9) decrease in systolic blood pressure recorded at the clinic, and with a 9.3-mm Hg (95% confidence interval, 2.1 to 16.5) decrease recorded by ambulatory measurement. We conclude that pathological processes initiated in utero become self-perpetuating in adult life and lead to hypertension. Among elderly people with established hypertension, these processes have a strong effect on blood pressure levels, because they are processes that do not respond well to treatment.