Factor VIII procoagulant activity (VIII-C), related antigen (VIII-AG), and von Willebrand factor (VIII-vWf) were measured in 38 patients with chronic renal failure (CRF), in 19 patients with normal renal function from a general medical ward, and in 17 normal subjects. Wide variation in the levels of factor VIII (FVIII) functions were found in the group of CRF patients. There was no correlation between the levels of the three individual activities of FVIII in renal disease. The levels of the three functions were elevated in the patient controls and no significant differences were found between the two groups of patients, although both groups were significantly different from the normal subjects. Increased electrophoretic mobility (EM) was detected in 22 of the 38 patients with CRF (58%) compared to seven of the 19 patients controls (37%). These studies suggest caution in interpretation of changes in FVIII status in patients with other clinical disorders, such as liver disease.