The pathogenesis of pulmonary veno-occlusive disease (PVOD) is not known. The diagnosis of PVOD frequently relies on its histological changes since it is often difficult to distinguish clinically from primary pulmonary hypertension. This study carried out a systematic analysis of the pulmonary venous and arterial remodelling that occurs in PVOD (n=5) and compared these changes to two other diseases affecting the pulmonary veins, mitral stenosis (MS; n=6) and fibrosing mediastinitis (FM; n=2), using established morphometric techniques. In PVOD, pronounced intimal and adventitial thickening were noted in veins of all sizes and arterialization of veins >50 microm external diameter was found. Similar changes were evident in the arterial wall, but intimal thickening was less severe than in the veins and medial thickening was more pronounced in arteries <300 microm external diameter. Eccentric intimal fibrosis of the veins was also noted for the first time in PVOD, although this feature occurred less frequently (approximately one third) than in MS. Less pronounced structural remodelling was also encountered in the veins in cases of MS and FM. The severity of the venous changes in PVOD may aid its diagnosis and lend insight into its pathogenesis. However, the similarity of the vascular changes in each form of venous hypertension also suggests that pathology alone may not always differentiate between these disease states. The similarity of the vascular changes in the three forms of venous hypertension suggests that, as in pulmonary artery hypertension, pressure, per se, is one of the triggers to vascular remodelling.