Congenital muscular torticollis (CMT) is called 'pseudotumor of infancy' because it is commonly discovered and diagnosed within 14∼28 days after birth as a sternomastoid tumor. We report two cases of CMT which presented as head tilt without any palpable neck mass immediately after birth but later developed into sternomastoid tumors. Serial ultrasonography confirmed increased echogenicities of the sternocleidomastoid muscles. We think that these findings are prodromal signs of sternomastoid tumors. Close physical examination by a neonatologist is crucial for an early diagnosis of CMT. Neonates with head tilt and increased ecogenicity of the sternocleidomastoid muscle on ultrasonography should be carefully followed up to see whether neck mass develops later. The authors suggest that CMT is not a static entity but rather one that progress to mass alteration after birth. (J Korean Acad Rehab Med 2010; 34: 591-594)
