Cystic hygroma is a congenital lymphatic malformation involving mainly the head and neck region in children before age 2 years old. It is seen rarely in adults. We report a 53 year-old female referred by a local physician as cystic goiter. Sonography showed a cystic mass lateral to thyroid and carotid artery. Computed tomography depicted a low density lesion posterior lateral to sternocleidomastoid muscle with compression of internal jugular vein. Complete excision was performed without recurrence. Cystic hygroma must be considered in the differential diagnosis of a cystic mass of the neck in adults. Complete excision without sacrifice of vital structure is the treatment of choice.