Factor VIII/von Willebrand factor antigen and von Willebrand factor activity (ristocetin assay) were studied in 12 patients in renal failure. A dramatic increase in both activities was observed (antigen 315 +/- 30 per cent in patients verus 104 +/- 9 per cent in control subjects; activity 402 +/- 48 per cent in patients versus 111 +/- 5 per cent in control subjects; p less than 0.001 for both). Since von Willebrand factor is thought to play at least a facilitative role in the development of arteriosclerosis, these increased activities may contribute to the premature arteriosclerosis reported in patients with chronic renal failure undergoing dialysis.