The ulnar tunnel (Guyon's canal) is a fibroosseous tunnel along the anteromedial portion of the wrist that contains the ulnar nerve and artery. As with the adjacent carpal tunnel, its main clinical significance is that it may cause nerve compression. The purpose of this study was to determine whether the anatomy of this area could be depicted in sufficient detail by MR imaging for MR to be useful in the evaluation of patients with ulnar neuropathy. MR studies of 36 wrists of volunteers were reviewed with attention to the size and shape of the canal, its anatomic boundaries, the presence of anomalous muscles, the size and bifurcation of the ulnar nerve, and the presence of a fibrous or muscular arch overlying the deep motor branch of the ulnar nerve. T1-weighted transverse MR images of 3-mm-thick sections were obtained by using either an extremity coil or dedicated wrist coil. Excellent anatomic delineation was achieved. The boundaries and shape of the canal varied from proximal to distal, but no statistical differences were present in the mean cross-sectional area of the canal. Anomalous muscles were present in the canal in nine (25%) of 36 wrists; six (67%) of the nine were bilateral. The ulnar nerve had a mean diameter of 3 mm and bifurcated an average distance of 12 mm from the proximal margin of the pisiform bone. Delineation of the fibromuscular arch at the origin of the flexor digiti minimi brevis muscle was limited by imager resolution, but 50% were judged to be fibrous and 50% to be muscular. Our results show that MR images depict the ulnar tunnel in excellent detail. Since those structures associated with ulnar neuropathy are clearly delineated by MR, the procedure should be useful in the evaluation of patients suspected of having ulnar nerve compression within the tunnel.