Levels of factor VIII/von Willebrand factor were measured in 105 patients affected by glomerulonephritis either of primary origin or associated with systemic diseases. The median plasma concentration of factor VIII-related antigen was significantly higher in the patients than in healthy controls. Amongst patients with primary glomerulonephritis, higher levels were observed in minimal change nephropathy than in other types. In patients with secondary glomerulonephritis, plasma factor VIII-related antigen was particularly elevated in lupus nephritis and in renal amyloidosis. Altogether, patients with the nephrotic syndrome showed higher levels than patients with lesser degrees of proteinuria. A negative correlation was found between the plasma concentration of factor VIII-related antigen and that of serum albumin but no correlation was observed with the extent of proteinuria. In 48 patients, plasma factor-VIII procoagulant activity was also measured and found to be elevated to the same extent as factor VIII-related antigen in the majority of cases. The urinary excretion of factor VIII-related antigen, evaluated in 72 patients, was found in variable amounts in 31 cases without any correlation with proteinuria. Glomerular deposits of factor VIII-related antigen were an uncommon finding. After 24 months, there was no clear evidence of an unfavourable association between increased plasma levels of factor VIII-related antigen and the course of the disease. Our findings suggest that the measurement of levels of factor VIII/von Willebrand factor in plasma or urine must be interpreted with caution in predicting the clinical course of patients affected by glomerular disease.