The purpose of our study was to check whether the dosage recommended for the low molecular weight heparin tested here, i.e., 50% of the corresponding unfractionated heparin dose, is adequate to prevent clot formation in the extracorporeal system. Sixteen dialysis treatments of 4-5 h were given to each of six chronic dialysis patients. In dialyses 1, 2, 15 and 16 unfractionated heparin (initial dose 35 IU/kg, continuous dose 20 IU/kg/h) was given, and in dialyses 3-14 low molecular weight heparin (initial dose 17.5 anti-Xa U/kg, continuous dose 10 anti-X U/kg/h). At these dose levels of low molecular weight heparin, clot formation occurred in the extracorporeal system in five of the six patients, despite the fact that the plasma anti-Xa level of 0.5 U/ml recommended by the manufacturer had been attained. For this reason the continuous dose of low molecular weight heparin had to be raised to approx. 80% of the corresponding continuous dose of unfractionated heparin. A plasma anti-Xa level of 0.7 U/ml is necessary to prevent extracorporeal clot formation.