The effectiveness of 4 X 500 ml dextran-40 infusions (Rheomacrodex) for the prevention of postoperative deep vein thrombosis was evaluated in a prospective, controlled, randomized study in urological and general surgical patients. The 125I-fibrinogen test was performed daily in all patients. 36 of 100 control patients developed deep vein thrombosis, but only 20 out of 92 (21.7%) did so in the dextran group (P less than 0.05). The number of patients showing bilateral deep vein thrombosis was significantly reduced under dextran prophylaxis (P less than 0.01). No side effects of dextran prophylaxis were seen.