Thenar hypoplasia can be an isolated defect, as in Cavanagh's syndrome, can be present with cardiac (Holt-Oram syndrome) or eye (Okihiro's syndrome) disorders, or can be associated with hand anomaly, as in Haas's malformation. Vascular abnormality may be associated with thenar hypoplasia, which has been demonstrated in Okihiro's syndrome. Cavanagh's syndrome is a rare anomaly of the upper extremities that presents with unilateral or bilateral hypoplasia of the thenar eminence. Typical clinical, radiographic, and electrophysiologic findings emphasize the diagnosis. Differentiation from carpal tunnel syndrome is important to prevent unnecessary intervention. Electrophysiologic and radiographic findings are necessary tools for the physician to establish a correct diagnosis and make an appropriate referral. Because of its rarity, we present the case of an 8-year-old girl with this syndrome.