In a prospective randomised trial, two groups of 20 patients each were compared. The first group received low dose heparin (t.i.d.) and the second group received low dose heparin (t.i.d.) combined with aspirin. The incidence of deep vein thrombosis (DVT) were determined using both radioactive fibrinogen uptake test and venography. The correlation between the two methods of diagnosis was better than 90%. There was no significant difference in the incidence of DVT between the two prophylactic regimen. A significant tendancy towards increased bleeding in observed with the combination of low dose heparin and aspirin.