F VIII coagulant, F VHI-related antigen and F VIII ristocetin cofactor activity were significantly increased in 68 patients with various chronic renal diseases. All three F VIII functions correlated generally well with each other. A striking relationship between some F VIII activities and serum creatinine was detectable in patients with glomerulonephritis and kidney transplants, with mild or moderate renal insufficiency. This correlation was no longer present in terminal renal failure. The results suggest that in initial stages of renal disease elevated F VIII levels may be attributable to glomerular endothelial damage. In terminal renal failure, however, increased F VIII concentrations seem to result from nonspecific causes related to uremia such as acute phase reactions.