The authors have attempted to define the exact borders of the root exit zone (RExZ) of the facial nerve, measure the distribution of myelin histologically, and examine the relationship between contact vessels and the RExZ. Seventy-five facial nerves were obtained from brainstems excised from 44 adult patients at autopsy. The arteries and veins associated with the facial nerve were counted and measured. The facial nerves, associated vasculature, and adjoining portions of the brainstem were then removed en bloc. These tissues were serially sectioned and stained, and a photomicrograph of each section was obtained. The distribution of myelin on each section was measured from the upper edge of the supraolivary fossette, and the relationship between contact vessels and the RExZ examined. The lateral transitional zone of the facial nerve began 8 mm distal to the upper edge of the supraolivary fossette (root exit point RExP) and had a mean length of 1.9 mm. The root detachment point (RDP) of the facial nerve at the medial side was located very close to the beginning of the medial transitional zone. In more than 80% of the nerves that were examined, vascular structures compressed the central glial myelin of the nerve. The authors propose the use of the terms "RExP," "RDP," and "transitional zone," instead of RExZ, which cannot be well defined. The RDP appears to be a good landmark for use during microvascular decompression.