Present investigations regarding the pathogenesis of chronic venous insufficiency (CVI) are focusing on microvascular changes. The aim of our investigation was to examine the correlation between the thickness of pericapillary type IV collagen depositions, basal membrane alterations and transcutaneous oxygen tension (TcPO2) in CVI-patients. Histochemical and immunohistochemical investigation of the capillary basal membrane was performed on 15 biopsies from normal controls, as well as 30 patients with CVI stage I and III (classification by Widmer & Stähelin). In all subjects TcPO2 was measured just prior to biopsy procedures in exactly the same area where the specimen was subsequently excised. The microscopically measured thickness of the collagen IV layer and the basal membrane was increased significantly in patients with CVI. Specimens from normal controls showed a collagen IV layer thinner than 0.1 micron. Patients with CVI stage III revealed strong collagen IV depositions between 0.2 and 0.3 microns. Comparison between TcPO2 and histological findings in the measured ares showed oxygen pressure varying from 62 mmHg (SEM 4.94 mmHg) in normal controls, down to 13 mmHg (SEM 3.39 mmHg) in patients with thick collagen IV layers. Besides an increased collagen IV layer, microvascular thrombosis and a thickened basal membrane have to be considered for impaired capillary perfusion.