The ulnar tunnel syndrome is a relatively uncommon condition resulting from compression of the ulnar nerve as it passes through the loge de Guyon, or ulnar tunnel, in the wrist. The syndrome may cause weakness and atrophy of the intrinsic hand muscles and loss of sensation in the ulnar nerve distribution of the hand. The most common cause of ulnar tunnel syndrome is chronic occupational trauma, either from pressure exerted over the hypothenar eminence by a tool or from the use of the heel of the hand as a hammer. Nonoccupational etiologies include congenital, inflammatory, neoplastic, vascular, metabolic, degenerative and traumatic disorders. Diagnosis often requires neurophysiologic evaluation, in addition to a careful history and physical examination. Conservative treatment aimed at protecting the ulnar tunnel from trauma frequently is effective. Occasionally, surgical decompression of the ulnar tunnel is required.