We present a case of an unusual cause of ulnar pain on a 9-year-old patient. The patient had pain on the ulnar side of the wrist after a fall. MRI showed a poorly defined lesion on the ulnar nerve at Guyon's canal. The initial diagnosis was traumatic neuropathy. Following conservative treatment of symptoms for one year, the pain started again. Therefore, a new MRI was performed were progression of the lesion was observed. Excision of the lesion was performed and the specimen sent for pathologic analysis. The diagnosis was of microcystic lymphatic malformation. The patient had a satisfactory evolution, with complete resolution. This is the first description of a microcystic lymphatic malformation in an intraneuronal location.