Figure 1. A 29-year-old right-handed woman with a five-year history of a slowly enlarging mass on the left palm had a 2-by-2-cm mass over Guyon's canal in the left wrist that was firm and movable. Preoperative magnetic resonance images (Panels A and B) demonstrated that the mass involved the ulnar nerve (arrows). Careful microscopic dissection showed that the mass arose from a single fascicle of the ulnar nerve (Panel C). The mass was sent to the pathologists and found to be a schwannoma. The patient had normal ulnar sensory and motor function postoperatively, with no evidence of recurrence.