In case I, age 10 mos., with extrapulmonic dilatation of the pulmonary artery, and case 2, age 6 mos., with both extra- and intrapulmonic dilatation, the arterial defects are ascribed to congenital factors which are respectively pulmonary fibrosis and perforate interven-tricular septum. Cases 3 and 4, aged 5 and 22 yrs. respectively, exhibit subintimal thickening of the pulmonary artery and its branches. Infection is excluded, some inherent factor, probably familial in case 3, being the causative factor of the lesion. Case 5, age 39, with marked obliterating pulmonary endarteritis, is definitely infective, syphilitic, in origin.. Dyspnoea, pronounced cyanosis and edema characterize all the noncongeni-tally produced cases. Loss of elasticity of the pulmonary arteries bears a direct relation to the degree of cyanosis.