Hydroxychloroquine sulfate appears to be a safe drug with no apparent hemorrhagic or other complications when used pre- and postoperatively in the manner described. With the criteria for thromboembolism of abnormal impedance plethysmography and confirmatory clinical signs, protection seems to have been afforded by hydroxychloroquine sulfate. Since these studies support prior extremely favorable results, further clinical trial with parenteral hydroxychloroquine sulfate seems indicated.