To see whether or not the fibrin-stabilizing factor is involved in the pathogenesis of renal damage, we analyzed by IF the glomerular deposition of factor XIII (subunits A and S) in 161 patients with various renal diseases. In 4 out of 5 cases of thrombotic microangiopathy (80%), F XIII deposits were found in a continuous subendothelial pattern, in association with deposition of fibrinogen and FDP, suggesting the occurrence of intraglomerular coagulation. In 22 out of 45 patients with membranous GN (idiopathic or SLE-associated), F XIII deposits were found along the capillary walls in a subepithelial location. These findings were not correlated with the presence of particular histological or clinical features, nor with IF positive for fibrinogen, FDP and factor VIII, suggesting alternative pathways of fibrin formation or local collagen synthesis. Finally, in proliferative GN, either idiopathic (acute post-infectious and membranoproliferative) or systemic (SLE and vasculitis), as in other glomerular and non-glomerular diseases, the presence of F XIII deposits was negligible, even in cases positive for fibrinogen, FDP and factor VIII.