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Introduction: Entrapment of the neurovascular bundle in the neck may occur in scalene triangle and costoclavicular region; by abnormal muscular architecture. Case report: During routine dissection we found an anatomical variation of muscular boundaries of posterior triangle of the neck. The upper part of trapezius is broader and is attached to medial half of superior nuchal line and is mostly fibrous in origin. Upper part of sternocleidomastoid is also broader and is attached to the lateral half of superior nuchal line. As a result, both trapezius and sternocleidomastoid muscles are in contact forming a single muscle sheet in the upper half of the neck. At their junction the posterior external jugular vein was descending to enter the posterior triangle. The apex of the posterior triangle does not extend above the middle of the neck. Attachments of the two muscles to the clavicle is more extensive and results in the reduction of the size of the posterior triangle. Both the muscles had greater connective tissue components between the muscular fasiculi. Conclusion: The cleido-occipitalis part of the trapezius in our case was larger and it over laps the third part of subclavian artery and lower trunk of brachial plexus. This can entrap the lower trunk of brachial plexus leading to "thoracic outlet syndrome". The upper part of brachial plexus is overlapped by the upper expanded part of sternocleidomastoid, it can compress the plexus, leading to "cervical outlet syndrome". Chronic tension type headache usually arises from the trigger points in the upper half of the trapezius muscle, because of the fusion of trapezius and sternocleidomastoid muscles in their upper half. It may also arise from certain trigger points in the sternocleidomastoid muscle. Such abnormal muscular variation may lead to torticollis of the neck and subluxation of the atlantooccipital joint on that side [ABSTRACT FROM AUTHOR]