Sternocleidomastoid muscle fibrosis has been recognized for centuries, but its pathogenesis and treatment remains controversial. Pseudotumor of infancy is a firm fibrous mass in the sternocleidomastoid muscle appearing at 2 to 3 weeks of age. Congenital muscular torticollis is less common and appears later in life. Pseudotumor and congenital muscular torticollis probably represent different manifestations of sternocleidomastoid muscle fibrosis. Pseudotumor will usually resolve with conservative therapy; however, some patients will subsequently develop torticollis. Congenital muscular torticollis usually requires surgical release of the sternocleidomastoid muscle to achieve a good cosmetic result and to prevent plagiocephaly, facial asymmetry, and scoliosis. This report provides guidelines for the management of congenital muscular torticollis and pseudotumor of infancy based on the authors' experience and review of the medical literature. Representative case histories from the neonate through the adult are presented, and the pathogenesis, diagnosis, treatment, and prognosis are discussed.