The files of the Mayo Clinic were reviewed for records of patients who presented with unilateral involvement of one or more of the last 4 cranial nerves without evidence of involvement of other cranial nerves or long-tract signs. Cases were not included when there was evidence of widespread malignant neoplastic disease that could be expected to cause the neurologic disease. Twenty-nine cases were thus selected in which the etiologic diagnosis of cranial-nerve lesion was not obvious. Fifteen of these patients proved, on complete examination, to have neoplasms. Of the other 14, several had disease which might explain the neurologic findings (tuberculosis, syphilis, diabetes), but, in sone, no etiologic factors could be discovered. Some of the 14 patients had spontaneous recovery. The anatomy and physiology of the jugular foramen are reviewed in detail in an attempt to express some of the factors that may help one make a more precise anatomically localized diagnosis.