The aetiology of branchial cysts is unknown. It is possible that they arise by more than one mechanism. The major theories are that they originate either from the branchial apparatus or from lymphoid tissues. A retrospective review was conducted of 61 cases occurring over a recent 14-year period. Eighty-five percent were diagnosed after the age of 10 years, 80% occurred in the 'classical' position, 80% had a squamous epithelial lining and 87% had lymphoid tissue in the wall. The clinical and histological features strongly support the lymphoid aetiology theory for the majority of branchial cysts.