A review of the literature as well as recent surgical observations indicate that hemifacial spasm may be caused by normal or pathological vascular structures that cross-compress the facial nerve. The critical area of compression is invariably found at the brain stem exit zone of the seventh cranial nerve. In this area the central glial investment of the facial nerve changes to peripheral or schwannian myelin. It is suspected that this anatomical junction zone may be of pathophysiologic significance when directly compressed or irritated. A retromastoid craniectomy and vascular decompression operation is highly successful in relieving hemifacial spasm while at the same time preserving facial nerve function. This is in contrast to most commonly used destructive operations for hemifacial spasm. Microsurgical techniques, however, must be employed or high morbidity and mortality may occur from the retromastoid approach.