We examined the blood pressure of children with and without a family history of hypertension in a longitudinal study. Supine blood pressures were first measured in schoolchildren (mean age 8 years) in 1978 and then on nine more occasions until 1986. Blood pressures of parents were measured in the seated position and their medical histories were obtained in home interviews carried out between 1978 and 1979. Children with a family history of hypertension had a higher mean systolic blood pressure (SBP) at the first screening compared to children without such a family history. This difference persisted at each of the succeeding nine school visits. The parents in hypertensive families had a lower income, greater body weight, were less well-educated and were more likely to be black than parents in families without a history of hypertension. Mothers in hypertensive families were more likely to have a history of heart disease and elevated blood pressure during pregnancy than mothers in normotensive families. The correlations between blood pressure of mothers and their children tended to be higher than those between fathers and their children. Elevated blood pressure emerges well before adolescence among children with a family history of hypertension and the family environment appears to play an important role in its development.