Congenital muscular torticollis (CMT) is characterized by shortening or excessive contraction of the sternocleidomastoid muscle (SCM). The main purpose of this study was to evaluate the feasibility of quantifying SCM stiffness using acoustic radiation force impulse (ARFI) sonoelastography in infants with CMT. Twenty infants with an SCM thickness greater than 10 mm with or without involvement of the entire SCM length (limitation of neck rotation passive range of motion [PROM]: group 1S >30°, group 1M = 15°-30°) and 12 infants with an SCM thickness smaller than 10 mm with or without involvement of any part of SCM (group 2) were included. The SCM thickness was measured using real time B-mode ultrasound, and the local SCM shear wave velocity (SWV) and subcutaneous fat layer using ARFI sonoelastography. The neck rotation PROM was significantly greater in group 1S (36.5° ± 5.3°) than in group 1M (18.8° ± 4.9°; P < .01); the SWV of the SCM in the affected side (2.96 ± 0.99 m/s) was significantly higher than that in the unaffected side (1.50 ± 0.30 m/s; P < .01) in group 1. The SWV of the SCM was significantly higher in group 1S than in group 1M. There was significant correlation between the degree of PROM deficit of neck rotation and the SWV of the affected SCM (r = .75; P < .01) in all infants. This study revealed a difference in the SWV of the affected SCM in relationship to the limitation of neck rotation PROM in infants with CMT, if there was no difference in SCM thickness among infants.