Four theories have been suggested to explain the aetiology of lateral cervical cysts. Ascherson (1832) suggested that the cysts arose due to incomplete obliteration of branchial cleft mucosa, which remained dormant until stimulated to grow later in life. His (1886) suggested these cysts were vestiges of the precervical sinus. Wenglowski (1912) believed lateral cervical cysts developed from the third pharyngeal pouch (thymopharyngeal duct). A number of investigators during the 19th century noted the close relationship between lateral cervical cysts and lymphoid tissue (Lucke, 1861). Luschka (1848) suggested that cystic degeneration of cervical lymph nodes was the mechanism by which lateral cervical cysts were formed. This theory received little support until King (1949) studied the histology of a large number of lateral cervical cysts and concluded that these cysts resulted from cystic transformation of cervical lymph nodes. The evidence for and against these theories of aetiology is discussed. The debate is centred on a study of 20 patients with lateral cervical cysts operated on in the Department of Otolaryngology, Bedford Hospital, between January 1986 and December 1991. In all twenty cases the wall of the cyst was found to be composed of lymphoid tissue, histologically identical to the present in lymph nodes. The mean age of presentation was 31 years, and in no case was a tract or cord found which connected the cyst to the skin or pharynx. The evidence strongly suggests that lateral cervical cysts develop from the cystic transformation of cervical lymph nodes. Mechanisms by which this may occur are discussed.