1. A case is described of severe birth injury to the sternomastoid muscle in a breech-delivered two-day-old infant. The affected muscle showed widespread haemorrhage, fragmentation and necrosis of its fibres, and disruption and disorganisation of the endomysial sheaths.

2. Disruptive muscular trauma of this type is known to lead to florid fibroblastic proliferation with formation of a large mass of scar tissue. It is suggested that the "sternomastoid tumour" of infancy develops as a sequel to such trauma occurring during birth.

3. The theories of birth trauma and of ischaemia, in the etiology of the "sternomastoid tumour" of infancy and of congenital muscular torticollis, are not mutually exclusive but may be complementary, the circumstances causing the trauma also leading to ischaemic damage.