Femoral vein cannulation (FVC) with 8.5F Swan-Ganz catheter introducers allows expeditious intravenous access and rapid volume infusion; however, the incidence of associated iliofemoral deep venous thrombosis (DVT) is unknown. Trauma patients (n = 76) requiring unilateral FVC were prospectively entered into a study where they were resuscitated via FVC and serially evaluated with bilateral venous duplex sonography (VDS). The incidence of DVT in catheterized femoral veins was compared with that in uncatheterized femoral veins. Catheters were removed promptly and VDS was performed within 24 hours and weekly for 1 month. Iliofemoral DVTs were identified in 11 of the 76 patients (14%). Iliofemoral DVTs occurred on the cannulated side in 9 (81.8%) compared with 2 (18.2%) on the uncannulated side (p < 0.05). We conclude that the use of the 8.5F FVC is associated with an increased incidence of DVT and that despite its convenience, this technique should not be routine.