A rare patient may have fever of unknown origin (FUO) that is caused by pulmonary emboli, pelvic, or lower extremity venous thrombosis (DVT). This study reviews our experience treating patients with DVT that presented with a FUO over a two-year period. A FUO was defined as a temperature of greater than 38.4 degrees C on several occasions for at least three weeks duration that defied one week of hospital evaluation. DVT was considered as a probable cause of FUO if the following criteria were met: (1) a positive venous duplex image for acute DVT, (2) subsequent fever resolution within seven days of anticoagulation therapy. Five out of 89 patients (6%) met this criteria. Their mean age was 53 years. Four patients had iliofemoral DVT and one had femoropopliteal DVT. Two had lung scans, one was positive for pulmonary embolism, and the other was equivocal. All five patients responded to heparin therapy and their temperatures returned to normal within a few days. Venous thrombosis and/or pulmonary embolism should thus be borne in mind when patients with FUO are being evaluated.