Hematologic and coagulation studies were carried out during 12 heparin-free hemodialysis in 9 maintenance hemodialysis patients. Treatment employed a C-DAK 4000 cellulose acetate membrane hemodialyzer. Both hemodialyzer and blood tubing were periodically flushed with physiologic saline. No significant clotting of the hemodialyzers was encountered in uneventful dialyses. Platelet counts, platelet aggregation with ADP and epinephrine, antithrombin III and fibrinogen/fibrin degradation products were not significantly changed compared with pre-dialysis values. Fibrino-peptide A levels, elevated pre-treatment, demonstrated additional rise during dialysis. These findings support clinical experience that this anticoagulation-free method can be used safely and effectively to dialyze patients at risk for bleeding.