Invasive pulmonary haemangiomatosis is a recently described disease in which exceedingly thin-walled vessels of capillary or venous dimensions infiltrate the lung parenchyma and pulmonary blood vessels. The angiomatous vessels, of obscure origin, infiltrate the media and intima of muscular pulmonary arteries, pulmonary veins and venules. The occlusion of the veins and venules by the thin-walled vessels, and the reactive intimal fibrosis they provoke, leads to pulmonary capillary dilatation, collections of intra-alveolar siderophages, fibrosis of alveolar walls and osseous nodules. This secondary pulmonary veno-occlusive disease in turn leads to hypertensive pulmonary vascular disease. Hence invasive pulmonary haemangiomatosis represents a fourth cause of 'unexplained pulmonary hypertension', the other three being unexplained plexogenic pulmonary arteriopathy, recurrent pulmonary thromboembolism, and pulmonary veno-occlusive disease. Two previously reported cases of invasive pulmonary haemangiomatosis presented with recurrent haemoptysis and the gradual development of chronic respiratory insufficiency associated with diffuse infiltrates in the chest radiograph. In one of these cases a haemothorax had developed. Such clinical features may be of importance in coming to the correct diagnosis.