The prophylactic antithrombotic efficacy of a low molecular weight heparin was compared with a traditional unfractionated calcium heparin after orthopaedic surgery in 140 patients. Deep vein thromboses were detected in legs either by Doppler sonography or [125I]fibrinogen uptake tests in five (7.1%) and seven (10%) patients, respectively. The capacity of both drugs to prevent deep vein thrombosis was demonstrated. Compared with the control group, those who used low molecular weight heparin showed a significant increase of activated factor X inhibition and smaller increases in activated partial thromboplastin times. Tolerability of both drugs was good, with a low incidence of local side-effects.