By combining a polyacrylonitrile (PAN) -polyethyleneoxide (PEO) membrane with ionically heparin-bound catheter, tubing, and module header, a totally antithrombogenic continuous ultrafiltration system (ACUS) was developed. Antithrombogenicity of PAN-PEO hollow fiber membrane was considered to be due to the presence of highly concentrated PEO near the inner surface of the membrane wich has a finely dispersed microstructure. ACUS was clinically applied to 24 patients without systemic anticoagulation and one PAN-PEO filter functioned for average 32 hrs without deteriorating their bleeding tendencies.