The etiology of 389 cases of partial or complete bilateral abductor vocal cord paralysis has been determined and classified. One hundred and forty-nine were infants and children 12 years of age and under; 240 were adults, age 13 and older. In the infants and children the paralyses were congenital in 82 cases, of which 43 were associated with other congenital anomalies, and 39 were without associated anomalies. Fifty-nine cases were considered acquired, most being secondary to underlying congenital anomalies, particularly the associated findings of meningomyelocele, Arnold-Chiari malformation, and hydrocephalus. Eight cases of paralysis in this age group were of undetermined etiology. Of the 240 adult cases of bilateral vocal cord paralysis. 138 followed thyroidectomy. Fifty-two cases were associated with various neurologic disorders, including poliomyelitis, Parkinson's disease, cerebrovascular accident, Guillain-Barré syndrome, multiple sclerosis, neoplasms and other miscellaneous neurologic conditions. Sixteen cases were due to malignant neoplasms of the neck and mediastinum. The remaining 34 cases constitute a miscellaneous group which includes foreign bodies, bilateral neck dissection, infection, congenital lesions, trauma, and idiopathic paralyses. The characteristic symptoms of bilateral abductor vocal cord paralysis include normal or near normal phonation with inspiratory stridor which may progress to complete respiratory obstruction. These symptoms are due to the stationary but flaccid midline position of the vocal cords which places them in a phonating position, where they both obstruct the airway and produce a faily clear voice or cry. This paradoxical combination of symptoms was frequently found to be responsible for a failure or delay in diagnosis.