THE PROBLEM of etiologic diagnosis of lesions of the ulnar nerve is commonly encountered by both general practitioners and specialists in disorders of the nervous system. Many causes of ulnar neuritis are well known and easily identified. Trauma along the course of the nerve, metabolic or toxic neuropathies, or compression of the cervical roots usually presents no problem in diagnosis. Degenerative disease of the motor nerves or muscles is usually evident and, in contrast to other causes, sensory changes are usually absent. Frequently, however, the etiology of the ulnar nerve dysfunction remains obscure despite careful history, examination, and x-rays. Therefore, two cases of ulnar nerve dysfunction caused by a rather unusual mechanism will be described. Their interest lies not only in the peculiar etiology but also in the complete absence of associated sensory loss, the only finding being that of atrophy and weakness of the muscles supplied by the ulnar