PURPOSE: To evaluate the relationship between sonographic findings and clinical outcome of fibromatosis colli. MATERIALS AND METHODS: Sonograms and medical records of thirty nine infants with fibromatosis colli were retrospectively reviewed for sonographic features (location, shape, AP dimension of the lesion, lesion-SCM ratio and echopattern) and follow-up data (duration of treatment, functional and clinical outcome after physical therapy). RESULTS: Lesions were located in either the lower two third (19cases) or lower one third (20cases) of the sternocleidomastoid muscle. Thirteen cases (33%) involved only the sternal portion of the sternocleidomastoid muscle while no cases involved only the clavicular portion of the sternocleidomastoid muscle was seen. Twenty-six (67%) cases involved both sternal and clavicular portions. Thirty-four infants had a mass in the sternocleidomastoid muscle while 5 infants had only diffuse muscle enlargement without a definite mass. The echogenicity of the lesion were hyperechoic in 20 cases, isoechoic in 9, hypoechoic in 3 and patchy echotexture in 7 cases. The duration of treatment was shorter, and functional result was more satisfactory in the group with the diffuse muscle enlargement. The longer duration of treatment was required in patients with a larger size, larger L-S (lesion-SCM) ratio and patchy echo-texture of the lesion. CONCLUSION: Sonographic evaluation of fibromatosis colli provided a good predictive guideline of the duration of treatment and result after physical therapy.