BACKGROUND : Decompression of the culprit artery causing hemifacial spasm (HFS), which passes between the facial nerve (cranial nerve [CN] VII) and the auditory nerve (CN VIII), can be difficult, especially if the artery compresses CN VII right after passing between the 2 nerves. Perforators or small arteries branching from near the compression site to adjacent structures can hinder the decompression process because such vessels can anchor the passing condition. The effect of such perforators or small arteries on the decompression process in such cases was investigated. METHODS : The culprit artery passed between any part of CN VII and VIII in 59 of 396 consecutive patients with HFS who underwent microvascular decompression. The culprit artery compressed CN VII right after passing between the 2 nerves in 22 of the 59 cases. Direction of the perforators or small arteries from near the compression site and direction of decompression of the culprit artery were analyzed in these 22 cases. RESULTS : Perforators or small arteries were observed in 20 cases, predominantly in the medial direction. The culprit artery was mobilized toward the petrous bone direction in most cases. No clear relationship was found between the 2 factors. CONCLUSIONS : Variation of curvature or tortuosity of the culprit artery and length of perforators or small branches may also have affected the decompression process and the directions. Adequate dissection near the compression site to obtain maximum mobilization of the culprit artery is necessary to achieve successful decompression in such cases.