A histological study was made of the pulmonary vasculature in two cases of haematite lung. Between fibrotic nodules in the lung the muscular pulmonary arteries showed the development of longitudinal muscle in the intima, a change thought to be associated with stretch and distortion of these vessels. In some arteries this muscular layer in the intima showed secondary fibrosis. No muscularized pulmonary arterioles were seen, indicating an absence of constriction of the terminal portions of the pulmonary arterial tree. Pulmonary arteries in fibrotic areas had become progressively engulfed by the fibrous tissue. The occlusive and obliterative vascular changes in 'haematite lung' are in reality those of silicosis which are brought about by the inhalation of silica particles with the iron sesquioxide. The only feature peculiar to haematite lung is the intense accumulation of iron-containing dust in and around the pulmonary blood vessels.