Ultrasound-guided peripheral nerve block (PNB) has become a popular anesthetic procedure. We report a case of an enlarged brachial plexus nerve noted on ultrasonographic images, as part of PNB, which was diagnosed postoperatively as Charcot-Marie-Tooth disease (CMTD), an inherited neurological disorder of the peripheral nerves. Although nerve enlargement is characteristic of demyelinating diseases such as CMTD, the use of ultrasonography in the diagnosis of neurological disorders is a developing area for neurologists and anesthesiologists can lack knowledge in this emerging field. Unusual nerve presentation on ultrasonographic images during PNB anesthetic procedures should be recognized as being indicative of underlying neurologic disorders. This case highlights that increased awareness of the diagnosis of underlying neurologic disorders by ultrasonography would assist the general practice of PNB in anesthetic medicine. This is especially important as underlying neurological conditions can have important consequences for patient-appropriate anesthesia and may inform best anesthetic practice. A new category, "neurological disorder on ultrasound image", should be introduced to PNB knowledge in anesthetic field.