Pathologic changes in the basement membrane (BM) of postcapillary venules (PCV) and capillaries in rheumatoid arthritis (RA) synovium were studied by immunoelectron microscopy, using a monoclonal antibody against human type IV collagen, C(IV)22, and by electron microscopic morphometric analysis. The sublining region of RA synovium was classified into lymphocyte-rich areas, transitional areas, and interstitial areas, according to their pattern of cellular infiltration. In lymphocyte-rich areas, the BM of the PCV and capillaries were minimally thickened; disruption of the lamina densa was seldom seen. Transitional areas, which contained macrophages, lymphocytes, and plasma cells, had numerous PCV and capillaries. The BM was markedly thickened and partially multilamellated, and there were many disruptions in the lamina densa. The BM contained degenerated endothelial cells and cell debris. On immunostaining of this BM with monoclonal antibody C(IV)22, type IV collagen stained heavily, mainly in the disrupted lamina densa; this indicates that the thickening was, at least in part, the result of an increase in BM collagen. In uninfiltrated interstitial areas, BM were moderately thickened and multilamellated, and showed few disruptions of the lamina densa; there were similar increases in type IV collagen, but cell debris was seldom observed. Measurement of the BM width, the ratio of BM width to vessel diameter, and the fraction of vascular cross-sectional area occupied by BM demonstrated that the thickness of the BM of both PCV and capillaries was greatest in transitional areas and was smallest in lymphocyte-rich areas (P less than 0.01). Since macrophages and macrophage-derived factors have been found to promote synthesis of BM collagen type IV, and since transitional and interstitial areas are rich in macrophages and histiocytes, respectively, it is suggested that these mononuclear cells and the factors secreted by them play a significant role in the thickening of the BM of PCV and capillaries in RA synovitis.