The rate of postoperative deep vein thrombosis was studied by the 125I-fibrinogen method in 35 patients after total hip replacement. 12 out of 18 patients treated prophylactically with hydroxychloroquine sulfate developed deep vein thrombosis compared with 11 out of 17 patients in the control group. Platelet aggregability was inhibited in vitro by high concentrations of hydroxychloroquine sulfate, but these concentrations were not obtained in vivo and platelet aggregation was not inhibited in patients treated with hydroxychloroquine sulfate (Plaquenil®)