A 26-year-old man presented with congenital left thumb macrodactyly. MRI showed diffuse enlargement of the left median nerve from the wrist to the digits, with particularly severe swelling of the thumb; US revealed the median nerve swelling comprised enlarged hypoechoic nerve bundles and increased hyperechoic areas around the nerve bundles. These typical cable-like and spaghetti-like appearances led to the diagnosis of fibrolipomatous hamartoma (FLH). Only debulking was performed for cosmetic reasons and enlarged nerves contiguous to the skin of the distal phalanx were cauterized and dissected. The diagnosis of FLH was confirmed pathologically. FLH is a rare, congenital disorder characterized by anomalous overgrowth of fibroadipose tissue between and around nerve bundles. Patients with median nerve involvement often present with carpal tunnel syndrome. Once its characteristic image is obtained, biopsy is not necessary.