The etiology of congenital muscular torticollis remains a mystery despite intensive investigation. Magnetic resonance imaging (MRI) scans of 10 infants with this condition showed signals in the sternocleidomastoid muscle similar to those observed in the forearm and leg after compartment syndrome. Cadaver dissections and injection studies defined the sternocleidomastoid muscle compartment. Injection studies and pressure measurements performed at the time of bipolar release in three patients with congenital muscular torticollis confirmed the existence of this compartment in vivo. Clinical review of 48 children with congenital muscular torticollis showed a relation between birth position and the side affected by the contracture. Because of the association of congenital muscular torticollis with other intrauterine positioning disorders, we postulate that head positioning in utero can selectively injure the sternocleidomastoid muscle, leading to development of a compartment syndrome. Congenital muscular torticollis may represent the sequela of an intrauterine or perinatal compartment syndrome.