This study of the clinical features of lesions of the cauda equina is based on an analysis of forty-five cases from the Surgical Division of the New York Neurological Institute. Twenty-eight of the patients had new growths which involved the roots of the cauda equina; in the remaining seventeen, no tumor was found and the cases were mostly recorded as inflammatory lesions. Two cases of extensively enlarged veins between the caudal roots, which were more or less completely excised, were included in the tumor group because the enlarged blood vessels encroached on the roots and acted mechanically as if they were new growths. In the seventeen patients with inflammatory changes of some or all of the caudal roots, exploratory laminectomy was performed either because a preoperative diagnosis of tumor had been made or because a new growth could not be excluded. In these patients, the roots were found to be