OBJECTIVE: We have occasionally observed during vein stenting for proximal iliac vein stenosis, the appearance of a more distal stenosis in the iliac vein that had not been initially observed before placement of the more proximal vein stent. In the present retrospective study, we aimed to document this observation. METHODS: We identified patients in whom changes in the area measurement and linear dimensions of the external iliac vein (EIV) were observed on venography and/or intravascular ultrasound (IVUS) after stent placement for chronic nonthrombotic iliac stenosis in the common iliac vein (CIV). The images of these IVUS scans were subsequently analyzed to determine the cross-sectional area, major axis, and minor axis measurements in the EIV, before and after placement of a proximal CIV stent. RESULTS: A total of 32 limbs with complete and quality IVUS and venography images available that allowed for measurement of the EIV before and after vein stent placement in the CIV were evaluated. The patient cohort was 55% men, with a mean age of 63.8 ± 9.9 years and a mean body mass index of 27.8 ± 7.8 kg/m2. Of the 32 limbs, 18 were left sided and 14 were right sided. Most (n = 12 [60%]) of the limbs had presented with venous-related skin changes (C4 disease). The remainder of the cohort had had active (C6 disease; n = 4 [20%]) or recently healed (C5 disease; n = 1 [5%]) venous ulceration and isolated venous-related edema (C3; n = 3 [15%]). The minimal CIV area before and after CIV stenting was 28.47 ± 23.53 mm2 and 196.34 ± 42.62 mm2, respectively. The minimal mean EIV cross-sectional area before and after CIV stenting was 87.44 ± 38.55 mm2 and 50.69 ± 24.32 mm2, respectively, a statistically significant reduction of 36.75 mm2 (P < .001). The mean EIV major axis and minor axis had both decreased similarly. The minimal mean EIV major axis before and after CIV stenting was 15.22 ± 3.13 mm and 11.13 ± 3.58 mm, respectively (P < .001). The minimal mean EIV minor axis before and after CIV stenting was 7.26 ± 2.40 mm and 5.84 ± 1.42 mm, respectively (P < .001). CONCLUSIONS: The results from the present study have shown that the dimensions of the EIV can change significantly after placement of a proximal CIV stent. Possible explanations include masked stenosis due to distal venous distention resulting from the more proximal stenosis, vascular spasm, and anisotropy. The presence of proximal CIV stenosis can potentially lessen the appearance, or completely mask the presence, of an EIV stenosis. This phenomenon appears unique to venous stenting, and the prevalence is unknown. These findings underscore the importance of completion IVUS and venography after venous stent placement.