Objective To explore the underlying anatomy of iliac vein compression syndrome (IVCS) using CT, and discuss the imageological index for subtype diagnosis and potential clinical significance. Methods We retrospectively analyzed the imageological data of 69 IVCS patients from July, 2009 to June, 2014. According to CT findings, patients were categorized to simple IVCS (sIVCS, the iliac vein is compressed by only the anterior iliac artery, n=22), lumbar degeneration?related IVCS ( dIVCS, besides the iliac artery, the lower lumbar degenerative changes including osteophyte, protruded disc, etc. pressed the iliac vein from the back, n=33) and other IVCS causes (oIVCS, including tuberculosis, inflammation and fracture hematoma of the lumbar spine, n=14); meanwhile, 69 age? and sex?matched subjects was set as a control group. Evaluated indexes were onset age, course of lower limb swelling and pain, size of the iliac vein tunnel ahead lumbar (IVTAL), lower lumbar spine forward curvature angle (LLSCA), iliac vein?pressed signs by lower lumbar degeneration, compression sites, scope of deep venous thrombosis and interventional therapeutic effect. The differences of those indexes of various groups were compared. Data as onset age, course of disease, sizes of IVTAL and LLSCA were evaluated using variance analysis of the general linear model, with Bofferroni test correction for multiple comparisons. Data as iliac vein?pressed site, scope of venous embolism and therapeutic effect were assessed by crossing table χ2 test. All statistical analyses were performed using SPSS17.0 software (SPSS company, Chicago, USA). Bilateral P≤0.05 were considered to be significant. Results The onset mean age was (61.5 ± 10.6) yr. in dIVCS, (42.3 ± 6.5)yr in sIVCS. and(53.1 ± 16.8)yr. in oIVCS with a significant difference ( F=11.030, P<0.01). Mean sagittal diameter of the IVTAL and LLSCA were(2.3±0.5)mm and(121.8±5.4)° in dIVCS and(2.5± 0.5)mm and(124.4 ± 3.9)° in sIVCS, respectively; which were smaller than those of control group [(6.4 ± 1.6)mm and(127.5 ± 7.3)° , respectively ] and oIVCS [(5.9 ± 2.3)mm and(129.5 ± 5.9)° , respectively ](F=125.275,P<0.01 for sigittal diameter and F=7.95,P<0.01 for LLSCA). The degenerative changes compressing the iliac vein of dIVCS had 41 sites of 33 patients which were respectively the discal forward protrusion or bulge (51%, 17/33), vertebral anterior osteophyte (50%, 16/33)and lower lumbar vertebrae slippage (19%,8/33). The compression sites of dIVCS located exactly anterior to the fifth lumbar(18/33, 56%), the intervertebral disk between the fourth and fifth lumbar(9/33, 26%), the right front (3/33, 9%) and left front (3/33, 9%) of the fifth lumbar, however, that of sIVCS sited the right front(14/22, 64%), exactly the front(4/22, 18%)of the fifth lumbar and exactly anterior(4/22, 18%)to the intervertebral disk between the fourth and fifth lumbar, with a significant difference of location distribution(χ2=19.305, P<0.01). In sIVCSs, deep vein thrombosis of all exceeded the iliac vein length and implicated the femoral vein by 27%(6/22)and the popliteal vein by 73%(16/22), while in dIVCSs 18%(6/33)patients had deep vein thrombosis in the iliac vein, 30%(10/33)in the femoral vein and 52%(17/33)in the popliteal vein, and in oIVCSs, 30%(4/14)patients had deep vein thrombosis in the iliac vein, 40%(6/14)in the femoral vein and 30%(4/14)in the popliteal vein, with a significant difference of thrombosis scope(χ2=9.28, P<0.01). 86%sIVCSs needed intravenous stent?implanted operation to obtain effective treatment, only 52%dIVCSs were performed stent?implanted operation for effective therapy, none of oIVCSs had stent?implanted operation before the iliac vein recanalization. Conclusion CT can precisely display the pathological anatomy features of different IVCS patients, which can potentially help clinicians plan accurate treatment strategy.