Introduction Evidence regarding ultrasound assessment of left common iliac vein diameter (LCIV) is limited. Extensive work is currently being undertaken worldwide on non-thrombotic iliac vein lesions to identify patients who may benefit from intervention to alleviate symptoms of chronic venous obstruction. Interventions include long-term stent implantation to improve vein diameter stenosis. This study aimed to assess a cohort of symptomatic venous patients and the diameter of the LCIV in these patients. Methods : Retrospective medical records review of all patients attending a tertiary vascular surgery centre who underwent a venous duplex ultrasound assessment between April 2017 and February 2018 were analysed for assessment of LCIV. Medical records of those patients with documented LCIV diameter were assessed over 18 months of follow-up. Results : A total of 672 (271 males, 401 females) LCIV diameter measurements were collected. The age of the patients ranged from 21 to 95 years (mean = 56.38). Median LCIV diameter overall was 7.64 mm (IQR 5.80mm–9.00 mm). 40 patients (6%) were reported to have a LCIV diameter measurement of < 4 mm, 8 (20%) male and 32 female (80%). 17 of these 40 patients (47.5%) were treated conservatively. Median LCIV diameter was 3.4 mm (IQR 2.5–3.7). 21 of these 40 patients (52.5%) underwent superficial venous intervention only, with a median LCIV diameter of 3.5 mm (IQR 3.2–3.7) and 2 out of these 40 patients (5%) underwent deep venous stenting (2/2 female – 100%), with a median LCIV diameter of 2.9 (IQR 2.9–2.9). No patients underwent both superficial and deep venous treatment in this 40 patient cohort. In those undergoing superficial venous intervention, 4 (19%) underwent repeat treatment. The two deep venous stenting patients underwent magnetic resonance venogram and venogram with intravascular ultrasound to allow stent placement, which confirmed a narrowed left common iliac vein. Primary stent patency at 18 months was 100%. Conclusion In this large study cohort of venous duplex assessments the median vein diameter was 7.64 mm and 40 patients out of 672 had a vein diameter smaller than 4 mm. 2 patients underwent deep venous stenting with primary patency of 100%.