54 patients with venographically verified deep venous thrombosis (DVT) were randomized to treatment with either intravenous infusions of 240 anti-Xa U/kg/12 h unfractionated heparin (UFH) or 240 or 120 anti-Xa U/kg/12 h of the low molecular weight heparin Fragmin. Repeated venographies showed improvement in 48% of the UFH-treated patients and 50 and 77%, respectively, in the Fragmin-treated patients. Progression was seen only in the UFH-treated patients and was observed in 11%. Two bleeding complications were seen in the Fragmin group in 2 patients receiving the very high dose of 240 anti-Xa U/kg/12 h. Anti-Xa activity in plasma and activated partial thromboplastin time (APTT) does not correlate in the Fragmin-treated patients. Fragmin was as effective as UFH in preventing the progress of thrombosis in DVT. In another study 120 anti-Xa U/kg Fragmin given subcutaneously 2 times daily to 13 patients with DVT resulted in adequate anti-Xa activity but with a tendency for accumulation of the Fragmin-induced activity. Subcutaneous injections of Fragmin 2 times daily also appears to prevent the progression of thrombosis effectively.