With the purpose of studying the relationship between the pathological gradings and changes of the hemodynamics as well as the collagen typing of the pulmonary arteries in pulmonary hypertension, 9 autopsy cases including 6 cases of unexplained plexogenic pulmonary arteriopathy, 2 cases of diffuse interstitial fibrosis of lung and 1 case of chronic embolic pulmonary hypertension were collected as the pulmonary hypertension group (PAH). The pathological lesions of PAH in pulmonary arteries were graded as follows: Grade I showed medial hypertrophy; Grade II medial hypertropy and intimal cellular proliferation: Grade III medial hypertrophy and intimal fibrosis or embolic occlusion: Grade IV the above lesions plus plexiform lesions with/without focal necrotizing arteritis. Immunohistochemical staining with antibodies against collagen type I and type IV was performed in the paraffin sections of lungs using PAP method. The result indicated that: 1) there were significant differences between the mean medial thickness in pulmonary arteries (6.1% in the normal lung group and 26.1% in PAH) and the density of pulmonary artery (22.6% arterioles/cm2 in the normal group and 44.6 arterioles/cm2 in PAH) respectively (P < 0.01). 2) there was positive correlation between the grades of PAH lesions and the mean pulmonary arterial pressure (r = 0.68 P < 0.01). 3) collagen type I fibriles increased in the old lesions and collagen type IV fibrils were dominant in the early stage of PAH (reversible lesion).