Summary. Iliac vein compression syndrome (May-Thurner syndrome – MTS) is an anatomically variable clinical condition in which the left common iliac vein is compressed between the right common iliac artery and the underlying spine. This anatomic variant results in an increased incidence of left iliac or iliofemoral vein thrombosis. It predominantly affects young women in the second or third decades of life with preponderance during pregnancy or oral contraceptive use. Although MTS is rare, its true prevalence is underestimated but it can be a life-threatening condition due to development of pulmonary embolism (PE). In this case based review the authors present three cases of MTS. All patients had been previously confirmed with PE, but despite they were admitted to hospital, diagnosed and correctly treated for PE and investigated for thrombophilia, the iliac vein compression syndrome was not suspected or investigated. With this presentation the authors would like to emphasize that MTS is mostly underdiagnosed, and it needs to be ruled out in left iliofemoral vein thrombosis in young individuals.