Biopsies of the two heads of the sternocleidomastoid muscle were studied in 9 children with idiopathic torticollis, from 8 months to 17 years of age, who were undergoing surgical release of the sternal and clavicular attachments on the side of the contracture. Extensive fibrosis involved mainly the sternal head. Nonspecific myopathic changes, also mainly in the sternal head, included cytoarchitectural alterations of muscle fibers, necrosis, and focal inflammation. Histochemical type grouping and grouped atrophy were extensive in some cases and present in all except one, but the clavicular head was predominantly involved. Denervation and reinnervation are common chronic features in idiopathic torticollis, probably secondary to entrapment neuropathy: the accessory nerve reaches the clavicular head by passing through the sternal head. Separate arterial supplies predispose to ischemia in the sternal head, resulting in focal myopathy and fibrosis.