Antithrombotic activity, necessary doses and effects on coagulation and lipid variables of the low molecular weight heparin derivative Fragmin were compared to unfractionated (UF) heparin in long-term multicentre trials. Results of more than 10,000 dialyses are reported. On the basis of preliminary studies, UF heparin and Fragmin doses were used that lead to anti-Xa activities of more than 0.5 U/ml. With this dose, sufficient antithrombotic activity was achieved with both heparins. Bleeding complications were not noticed. Partial thromboplastin time (PTT) and thrombin time were only marginally increased by Fragmin (5–8 s) in contrast to UF heparin (PTT 90–120 s, thrombin time 230–260 s). Surprisingly, the elevated levels of factor VIII strongly decreased during the 6-month treatment period with Fragmin and increased again during the following 6-month treatment period with UF heparin. Creatinine, urea, haemoglobin and transaminases did not change in both heparin groups: this excluded reduced dialysis efficiency or occult blood loss. Additionally, 15 patients with acute renal failure and high bleeding risk were dialysed with low doses of Fragmin (anti-FXa: 0.2–0.3 U/ml). No severe bleeding occurred. A continuous ambulant peritoneal dialysis patient with deep vein thrombosis was treated effectively with intraperitoneal application of Fragmin for 6 months without any problems.