Despite the intensive study that has followed early discoveries, pulmonary hypertension in its various aspects continues to be a problem in clinical medicine. The means by which certain functional or structural changes occur in the lung and increase its resistance to blood flow remain obscure. It is true that these changes may be studied in histological detail at autopsy but the part that they play in the living organ is difficult to define. This is especially true of cases classified under the heading of primary pulmonary hypertension, in which traditionally are listed the cases which we know least about, and which therefore remain etiologically indistinguishable among themselves. In the absence of a large communication between the pulmonary and systemic circulations to invite surgical repair or to produce cyanosis, the symptomatology and causes of death are different from the Eisenmenger group of pulmonary hypertension. Very high pulmonary pressures and resistances are required to produce symptoms and signs. Primary pulmonary hypertension is listed in a special category of Wood's classification (1956), since the obliterative, vasoconstrictive, or reactive lesions according to this classification may be involved. Obliterative lesions, however, are almost always to be found, and may be produced by intimal hyperplasia of small pulmonary vessels, thrombosis, or arteritis. Thrombosis may be primary or secondary to changes that occur in the vessel wall following inflammation or high pressure. A result of the latter may be vasoconstriction, and from this may follow muscular hypertrophy in varying degree. Primary pulmonary hypertension, as usually described, occurs predominantly in young women between the ages of twenty and forty, although cases of most ages and of both sexes have been reported. (Berthrong and Cochran, 1955; Brenner, 1935; Chapman et al., 1957; Cross and Kobayshi, 1947; Wade and Ball, 1957; Fleming, 1960; and Okuyama, 1958). The disease is about three times as common in women. It was surprising, therefore, to observe over a short period of time a number of cases of right ventricular hypertrophy in infants and children in the absence of known causes. The group under discussion occurred in later childhood, in distinction from a group of five cases in new-born children observed by ourselves, who died with right ventricular hypertrophy and right heart failure without any obvious cause, even in the lung structure.