A sternocleidomastoid tumor of infancy, also known as fibromatosis colli of infancy, is a rare benign mass in the muscle on the side of the neck. The tumor is usually on the right side, and is firm, measuring a couple centimeters in diameter. It appears in a newborn, usually between the second and fourth weeks of life.

Fibromatosis colli is an unusual cause of a neck lump in infants, but the most common cause of congenital muscular torticollis. Diagnosis can be made confidently with a clinical examination and ultrasonography, allowing early parental reassurance and avoiding invasive investigations.