Venous hypertension is considered a significant factor in the initiation of venous incompetence by dilating and weakening the vein wall (Bergan & Schmid-Schonbein, 2000). Pressure in the leg veins is predicted by height, weight and calf volume, and an association between body mass index (BMI) as a measure of obesity and the incidence and severity of venous disease has been noted in several population-based studies (e.g. Lee et al. 2003). Another means by which a large BMI could contribute to high venous pressure is by mechanical impediment to outflow from the legs because of abdominal obesity when sitting. To investigate this, we studied venous outflow from the legs in 18 normal and overweight women (age 40 ± 6 y, mean ± SD; BMI range 23-49) with no venous disease as assessed by ultrasound scan and CEAP classification. All participants gave informed consent for the study which was approved by the Dudley Local Research Ethics Committee. Femoral vein diameter was measured 15 cms above the patella using online edge-detection software applied to longitudinal ultrasound images and vein cross sectional area (CSA) was calculated. Measurements were made with subjects supine and when seated (trunk vertical with legs straight out), and related to several indices of abdominal obesity. A novel index, the ratio of sacro-abdominal wall/sacro-patella distance when seated (SA/SP) correlated with BMI (r=0.93, Spearman’s correlation coefficient) better than waist-hip ratio (r=0.35) and SA/SP was significantly related to the percentage of truncal fat (estimated by DEXA scan, r=0.90, p<0.003). SA/SP also predicted the increase on sitting from lying in groin tissue pressure measured by needle manometry (r=0.83, p<0.0005). Tissue pressure increase correlated with the increase in vein CSA (r=0.82, p<0.002). Vein CSA increased significantly more (p<0.05, Anova) in those with BMI > 30 (156±12%, mean ± S.E.M., n=6) than normal (BMI<25, 87±25%, n=4) or overweight (BMI 25-30, 54±30%, n=4) women. These findings suggest that abdominal obesity increases impedance to venous flow through the inguinal region when in the sitting position, which could elevate venous pressure and contribute to the later development of venous disease.