Previous studies have shown a perivascular hyaline thickening affecting restricted regions of the microcirculation in gingivitis and moderate periodontitis and in the pulpal vessels in chronic pulpitis. In the present study of the lesion of advanced periodontitis, immunostaining for type IV collagen and laminin demonstrated widespread deposition of basement membrane material, with manifest involvement of the venous network. Some vessels were associated with an increased deposition of both basement membrane proteins, while others showed preferential deposition of either laminin or type IV collagen. Immunostaining also revealed an extensive trabecular network of type 1V collagen throughout the affected gingival tissue that was not related to recognizable vessels but was co-extensive with less intense staining for laminin. This network was not associated with viable endothelial cells demonstrable by staining with the endothelial marker Ufex agglutinin (UEA-1). The results indicate extensive vascular pathology in advanced periodontitis that could explain the attenuation of the inflammatory reaction and the restricted ability to develop reparative granulation tissue in this disease.