Klippel-Trenaunay syndrome (KTS) is a congenital condition redefined by Oduber et al (2008) by the coexistence of vascular malformations and disturbed soft tissue or bony growth, including hypertrophy or hypotrophy in the same or opposite sides of the body. The anomalies may involve part of a limb, a whole limb, a limb girdle, or a hemibody. Vascular malformations may involve veins, capillaries, or lymphatics although venous or capillary malformations are essential for the diagnosis. Associated venous anomalies include dysplasia, valvular malformations, and varicosities. Congenital venous anomalies are often associated with disturbances of blood flow and should be considered as prothrombotic states. However, such anomalies are not considered in Wells scores and used to determine the risk for venous thromboembolism (VTE). We present the case of a male with unrecognized crossed dissociated form of KTS and unsuspected VTE. The pathophysiology and the treatment of VTE in KTS are discussed. We suggest physicians to be aware of KTS and that its recognition in a critically ill patient should prompt consideration for appropriate prophylaxis for high-risk category for VTE. Dedicated duplex sonography should be obtained if VTE is suspected. We also suggest a modification of the Wells scores to reflect the association of KTS and VTE.