OBJECTIVE: Neurosurgical procedures such as proximal brachial plexus repair, scalenotomy, and direct isolation of the proximal vertebral artery require a good working knowledge of the triangle of the vertebral artery. This deep triangle of the neck is bound by the subclavian artery and the anterior scalene and longus cervicis muscles. In addition to the vertebral artery, many important structures are found in this area, such as the ganglionated sympathetic chain and certain cervical spinal nerves. METHODS: Twenty formalin-fixed cadavers were used for this study. Dissection of this triangle was performed, and measurements were made not only of parts of its borders, but also distances from these borders to neurologically important structures within its confines, such as the C8 spinal nerve. RESULTS: In all specimens, the middle scalene muscle was noted to form part of the posterior wall of the triangle. The mean height of the triangle was found to be 3.2 cm, and the mean width of its base was 1.3 cm. We observed that the C8 spinal nerve had a mean distance of 1.2 cm inferior to the apex of the triangle and that the C7 spinal nerve was found inside the triangle in 5% of sides. If the phrenic nerve entered the triangle, it was never found more than 6 mm medial to the anterior scalene muscle. The vertebral artery always traveled intimately along the lateral border of the longus cervicis muscle, and its lateral edge ranged 5 to 8 mm medial to the medial edge of the anterior scalene muscle. CONCLUSION: The C7 spinal nerve was observed in the triangle of the vertebral artery. In addition, the posterior border of the triangle of the vertebral artery was clearly defined in this study, and the middle scalene muscle could be used as a landmark. These data, coupled with our quantitation of parts and structures within the triangle, may assist neurosurgeons who operate on this area of the neck.