To develop a continuous arteriovenous hemofiltration (CAVH) system, which does not need systemic anticoagulation, for patients of acute renal failure having bleeding tendencies, a totally antithrombogenic continuous ultrafiltration system (ACUS) was designed, which consists of an antithrombogenic polyacrylonitrile-polyethyleneoxide (PAN-PEO) hollow fiber membrane and ionically heparin-bound catheter, tubing, and module header. Antithrombogenicity of PAN-PEO membrane, which occupies more than 90% of total inner surface area of ACUS, was considered to be due to highly concentrated PEO near the inner surface of the membrane and the finely dispersed (less than 500 A) microstructure of the inner surface. ACUS was applied to 24 patients without systemic anticoagulation, and one filter worked for an average of 32 h without deteriorating their bleeding tendencies. Any significant changes in major parameters of biocompatibility during those treatments were not observed. More than 200 ml/h of ultrafiltrate was obtained even under very low mean blood pressure, less than 70 mm Hg. Based upon these results, ACUS was concluded to be suitable for mild and sustained treatment to control fluid and electrolyte balance in patients of acute renal failure with bleeding complications.