Anticoagulation with heparin is required in the management of the burn patient if their clinical course is complicated by venous thrombosis. Heparin therapy is commonly monitored by the activated partial thromboplastin time (APTT) but this assay can be unreliable in patients with acute inflammation because of an increase in plasma factor VIII levels that result in an underestimation of the heparin concentration. We report an example of heparin resistance that occurred in a patient who developed venous thrombosis following extensive second-degree burns. Heparin doses in excess of 60,000 units per day were required to produce a significant elevation in the APTT. The plasma factor VIII level was found to be markedly elevated to 455% and the plasma heparin concentration as determined by the anti-factor Xa assay was disproportionately elevated in relation to the APTT. Physicians treating patients with burn injury complicated by venous thrombosis should be aware of the potential development of factor VIII-related heparin resistance when large amounts of heparin are required to obtain a satisfactory elevation in the APTT. Measurement of the plasma heparin concentration will avoid excessive heparin administration and the serious bleeding which can result.