A 4 1 -year-old right-handed man complained of weakness, cold sensitivity, and signs of ulnar intrinsic paresis in his right hand. He denied having pain, numbness, or paresthesia. He had no other upper extremity problems or other health problems. Physical examination showed a healthy, thin man with hypothenar and interosseous wasting but with minimal clawing and normal function of ulnar nerve-innervated extrinsic muscles. Tinel’s sign could not be elicited over the entire course of the ulnar nerve or over the median nerve at the wrist. Allen’s test confirmed ulnar artery patency. Grip strength was diminished by one third and pinch strength by two thirds as compared with the normal hand. X-ray films were normal. Electrodiagnostic studies confirmed major interference with motor branches of the ulnar nerve, but sensory conduction was normal. Guyon’s canal was explored. We found a small, pale, atrophic muscle arising from the forearm fascia and attaching to the pisiform bone (Fig. 1). This muscle compressed the ulnar nerve, and after the muscle was divided we determined that there was a neural loop of the ulnar nerve with take-off