Sixty-eight patients with acute deep vein thrombosis were allocated at random to two treatment groups. One group (n = 33) received a fixed dose of 750 anti-Xa units of a low molecular weight heparin (CY 222 Choay Institute); the other group (n = 35) received standard heparin in doses of 500 IU/kg/24 h. Both treatments were given for 10 days in two daily subcutaneous injections. A second phlebography was performed on the last day of treatment. No haemorrhagic complication was observed in the group treated with CY 222, as opposed to three cases of haemorrhage in the group treated with standard heparin. The initial phlebographic score and the location of deep vein thrombotic lesions were the same in both groups. Angiographic improvement, with more than 30% thrombolysis, was obtained at the end of treatment in 64% of patients in the CY 222 group and in 65% of patients in the standard heparin group (NS). In 2 patients treated with standard heparin the second phlebography showed extension of the thrombosis. The initial score remained unchanged in 1/3 of patients in both groups. The activated partial thromboplastin time was prolonged (2 or 3 fold the normal value) in the standard heparin group and unchanged in the CY 222 group. Anti-Xa activity was significantly higher in the CY 222 group than in the standard heparin group. It is concluded that CY 222 and standard heparin were equally effective in patients with deep vein thrombosis. However, haemorrhagic complications were more frequent with standard heparin that with CY 222.