Background: Fibromatosis colli is a benign uncommon condition of infancy, usually manifested at the age of 2-4 weeks of life as a lump in the neck (mostly on the right side) or head tilt. It can be easily diagnosed by ultrasound (US). Case Presentation: A 3-day-old full-term male neonate was brought to the emergency department with a complaint of left side neck swelling. On examination, the child was normocephalic with open flat anterior fontanelle, normal head, throat and ears, the neck was not tilted with no apparent signs of trauma. Locally, a ½ cm mass which was firm, non-warm, and non-tender was felt on the left sternocleidomastoid without cervical lymphadenopathy. The patient was sent for US. Result: US examination showed a fusiform mass-like lesion within the left sternomastoid muscle, which showed similar echogenicity to the muscle and muscle striation within the lesion, with no abnormal cysts or suspicious features, suggestive of fibromatosis colli. Conclusion: Familiarity with fibromatosis colli is important to avoid extensive or invasive unneeded investigations and also to reduce anxiety of parents.

Keywords: Fibromatosis colli, neck lump, ultrasound, case report