UNLABELLED: Isolated iliac venous thrombosis (IIVT) is uncommon. Duplex ultrasonography of the iliac vessels is not recommended and not generally performed. OBJECTIVE: The purpose of this study was to determine the frequency of IIVT in a hospital recruitment population and to identify characteristic features of onset which might be associated with this localization in order to better target explorations. MATERIAL AND METHODS: The study included 18,297 patients referred for Duplex-ultrasonographic exploration of possible deep vein thrombosis of the lower limbs between January 1st 1994 and December 31st 2000. Selection of isolated iliac thrombosis, defined as the absence of retrograde extension to the common femoral vein, was made from the digitalized data recorded daily. The following factors were tested: sex, age, absence of clinical signs in the lower limb, presence of pulmonary signs. The raw odds ratios were calculated followed by construction of a multivariate logistic regression model. The circumstances of onset were retrieved from the patient's medical files. RESULTS: Isolated iliac venous thrombosis was discovered in 48 patients, i.e. 0.26% (95% CI 0.19%-0.35%) in the recruitment population and 0.82% (95% CI 0.61%-1.09%) among the 5827 patients with thrombosis. The common iliac was involved predominantly (35 out of 48). The left side predominated in women compared with men (24/36 versus 4/12) (p = 0.04). Specifically female circumstances (oral contraceptives, peri-obstetrical period) always led to a left localization. For the other identified circumstances (cancer, inflammatory bowel disease, orthopedic surgery, pelvic trauma), there was no predominant side. Variables explaining the multivariate model were sex, age (less than or more than 35 years), suspected pulmonary embolism, and age interaction with suspected pulmonary embolism. For women, the risk of IIVT was twice as high as for men (OR = 1.97, 95% CI 1.02-3.81). Young age was also a risk factor for IIVT and increased with suspected pulmonary embolism. The odds ratio for subjects under 35 varied from 4.20 (95% CI 1.79-9.84) without suspected pulmonary embolism to 35.01 (95% CI 14.78-82.89) with suspected pulmonary embolism. CONCLUSION: The incidence of isolated iliac venous thrombosis is very low (0.26) in this hospital recruitment population but reached 9.40% in young women under 35 with suspected pulmonary embolism. These age and sex characteristics are also the principal circumstances for the development of these thrombotic events (pregnancy, post partum period, oral contraception). Under these circumstances, it is necessary to carefully explore the iliac vessels with duplex ultrasonography.