We report the case of a 19-year-old man who presented with a 12-year history of progressive fatigue, feeling hot, excessive sweating, and numbness in the left arm. He had undergone multimodal imaging and was diagnosed as having Klippel-Trénaunay-Weber syndrome (KTWS). This is a rare congenital disease, defined by combinations of nevus flammeus, venous and lymphatic malformation, and hypertrophy of the affected limbs. Lower extremities are affected mostly. Conventional modalities for evaluating KTWS are ultrasonography, CT, MRI, lymphoscintigraphy, and angiography. There are few reports on multimodal imaging of upper extremities of KTWS patients, and this is the first report of an infrared thermography in KTWS.