The long-term patency of arteriovenous (AV) fistulas created for hemodialysis of renal-failure patients is usually measured in months, particularly when polytetrafluoroethylene (PTFE) material is interposed between the artery and vein. This is due to the rapid development of intimal hyperplastic lesions in the anastomosis region of the PTFE graft material with the vein. We studied the proliferative patterns in seven human AV fistulas removed at the time of fistula revision. Cell proliferation was determined by using an antibody to the proliferating cell nuclear antigen (PCNA), and specific cell types were identified by immunochemical reagents for smooth muscle cells, monocytes/macrophages, monocytes, lymphocytes, and endothelial cells. All venous segments exhibited a markedly hyperplastic intima. Vascularization of the intima and media by capillary-sized vessels was found. The main intimal cellular component was smooth muscle. Macrophages were usually seen around microvessels, and many also populated the perigraft region of the adventitia. In contrast to human atherosclerotic lesions, high rates of cell proliferation were observed in these fistulas. PCNA indices (percentage of cells that were PCNA positive [mean±SD]) were as follows: intima 17.7±11.3%, media 24±11.2%, and adventitia 20±11.6%. However, the distribution of PCNA-positive cells was not uniform. Instead, the PCNA index in microvesselcontaining intimal fields was five to six times that of avascular fields (28.9 ±10.6% versus 4.9 ±4.5%, respectively,p< 0.001). Double immunolabeling revealed a large proportion of PCNA-positive microvascular endothelial cells and surrounding pericyte-like smooth muscle cells, a few proliferating macrophages, luminal endothelial cells, and subendothelial smooth muscle cells, as well as smooth muscle cells without visual connection to either microvessels or the lumen. Thus, cell proliferation associated with neovascularization seems to be one of the major mechanisms of human AVfistulafailure.