We studied the pharmacokinetics of the low-molecular-weight heparin (LMWH) Fraxiparin in 6 patients with the nephrotic syndrome. Maximal plasma anti-Xa activity was obtained 5 h after a subcutaneous injection of 60 IU anti-Xa/kg. Anti-Xa activity was no longer detectable 24 h postinjection. These results are similar to those obtained with the same dosage in other clinical settings. The biological response to a daily subcutaneous injection of 60 IU anti-Xa/kg was studied for 8 days. No cumulative effect was observed. All the patients had an abnormal activation state of hemostasis mechanisms before treatment, as shown by high prothrombin fragment 1 + 2 and D-dimer levels, which were not decreased by Fraxiparin. It remains to be determined whether a higher dosage of LMWH might attenuate the hypercoagulability in these patients.