Heredity is the most important known factor in the development of arteriolar (essential) hypertension. Valuable evidence for this opinion can be secured by studying the blood pressure in a large number of hypertensive and nonhypertensive families, with an effort to obtain the blood pressure readings of all of the available relatives in two or more generations. This has been the purpose and method of the present study. A definite relationship between heredity and arteriolar (essential) hypertension and its complications was noted long ago by Dieulafoy, Raymond, Broadbent, Gowers, Allbutt1 and others. More recently, this relationship was further demonstrated by the results of systematic studies of family histories. In the studies, O'Hare, Walker and Vickers2 and Weitz3 showed that a history of cardiovascular disease is frequent among the relatives of hypertensive patients. Weitz3 showed in addition that there was an abnormally high incidence of elevated blood pressure