A description is given of a disease of infants occurring in Lhasa, Tibet at an altitude of 3600 m. Typically if affects infants who have been born at low altitude and subsequently brought to residue in Lhasa, and it is usually fatal within a few weeks or months. There is extreme medial hypertrophy of muscular pulmonary arteries and muscularization of pulmonary arterioles, together with dilatation of the pulmonary trunk and massive hypertrophy and dilatation of the right ventricle. The disease is distinct from acute or chronic mountain sickness and appears to be the human counterpart of 'brisket disease' in cattle.