Evidence for the usefulness of hydroxychloroquine as prophylaxis against thromboembolism after total hip replacement is examined. This agent causes reduction in red blood cell aggregation without prolonging the bleeding time in humans and, experimentally, reduces the size of the thrombus. There is a variably demonstrable reduction in platelet aggregation and blood viscosity in humans. After hip replacement, the venographic incidence of deep vein thrombosis is not influenced by hydroxychloroquine. The incidence of fatal pulmonary embolism appears to be reduced, but a controlled trial against placebo does not exist. There are no serious or irreversible side effects, and wound healing is not affected by the administration of this drug. Hydroxychloroquine remains the prophylaxis of choice in use at the Hip Center, Wrightington Hospital, England, 13 years after its introduction.