Objective: To review the ultrasonographic appearances of sternocleidomastoid masses in infants. Methods: Ultrasonograms and medical records of 10 infants with sternocleidomastoid masses were reviewed. Diagnosis was based on spontaneous disappearance of the masses in eight infants and the results of biopsy in two infants. Results: Eight infants had fibromatosis colli. Seven of these had diffuse enlargement of the sternocleidomastoid muscles, most of which was hyperechoic or mixed echoic. Only one infant with fibromatosis colli had a mixed echoic mass with diffuse muscle enlargement. Two other infants who had mixed echoic masses in the muscles turned out to have lipoblastoma and myofibroma. Conclusion: Biopsy should be done when (1) ultrasonography of an infant with sternocleidomastoid mass shows a mass of mixed echo and (2) clinical features of the infant are not characteristic of fibromatosis colli.