Objective: The aim was to assess the anatomical distribution of acute deep venous thrombosis (DVT) with a focus on iliofemoral DVT, and, in particular, to characterise thrombus in the common femoral vein (CFV) and the deep femoral vein (DFV). Methods: A one year prospective study including patients older than 18 years of age with an acute first time DVT according to ultrasound examination at one of three university hospitals in Copenhagen, Denmark. Thrombus location and extent were registered and divided into five segments: calf veins; popliteal vein; femoral and deep femoral vein; common femoral vein; and iliac veins and/or the inferior vena cava. Thrombus appearance of the CFV and the DFV (partial or occlusive) was examined in detail. Results: Acute DVTs were identified in 203 extremities in 200 patients (58% male). The median age of the patients was 68 years (range 19–92 years), and left-sided DVT was observed in 56%. Iliofemoral DVT was present in 54 (27.0%) patients. Thrombus involving the CFV but not the iliac veins (CFV group) was seen in 28 patients; the remaining 26 had involvement of the iliac veins (iliac group). Thrombus in the CFV was more likely to be occlusive in the iliac group than in the CFV group (77% vs. 4%; p < .001). Thrombus in the DFV was more often occlusive in the iliac group than in the CFV group (81% vs. 11%; p < .001). The DFV was free of thrombus in 12% of patients in the iliac group and in 64% of those in the CFV group. Conclusion: The presence of occlusive thrombus in the CFV and/or in the DFV pointed to a DVT also involving the ipsilateral iliac veins. Thrombosis of the deep leg veins extending into the CFV below the inguinal ligament was more likely to be partial in the CFV, mainly due to inflow from the DFV.