To study whether the activated clotting time (ACT) is a sensitive parameter to monitor anticoagulation with low molecular weight heparin (LMWH) during hemodialysis, ACT, polymorphonuclear granulocyte-elastase, and anti-factor Xa activity were studied during 30 dialysis treatments with LMWH (35 IU/kg body weight bolus; 10 IU/h/kg). Twenty treatments were performed with Hemophan, ten with polysulfone dialyzers. No clinically relevant clotting of dialyzers was observed, but minimal fibrin deposition was found more often in the Hemophan group (50 vs. 30%). Despite continuously elevated anti-factor Xa levels (Hemophan 0.49 ± 0.03, polysulfone 0.62 ± 0.01 IU/ml), a significant increase of ACT was only demonstrated 10 min after bolus application in the Hemophan group. Elevated polymorphonuclear granulocyte-elastase levels were demonstrated in the Hemophan group but were linked to the presence of minimal fibrin deposits and not to the dialyzer material. We conclude that ACT is not a sensitive parameter to monitor anticoagulation with standard doses of LMWH.