Although thrombomodulin (TM) in circulating blood is regarded as an indicator of vascular endothelial disorders, blood TM levels are also known to be affected by renal dysfunction. We measured plasma TM levels in primary glomerular disease (PGD) and lupus glomerulonephritis (GN) with the EIA method, and assessed the extent to which renal dysfunction and endothelial disorders contribute to plasma TM levels in these diseases. The plasma TM/serum creatinine (TM/Cr) ratio was significantly higher in lupus GN patients than in PGD patients or normal controls. A significant positive correlation was found between plasma TM and serum Cr levels in both PGD and lupus GN patients, but the slope (A) of the regression line (TM = A.Cr+B) in lupus GN patients was significantly steeper than in PGD patients. We conclude that plasma TM levels are greatly influenced by renal dysfunction, but that not only renal dysfunction but endothelial disorders may be an important determinant of increased plasma TM levels in diseases such as lupus GN.