We report an aggressive tumour in a 5-year-old girl causing facial disfigurement. Imaging confirmed a solid, diffusely enhancing mass at the right internal pterygoid muscle, infiltrating the adjacent bone. Surgical excision and reconstruction of the mandible were performed. Histology revealed aggressive infantile fibromatosis. No recurrence was noted 7 months later. Infantile fibromatosis may mimic malignancies and should be considered in aggressive mandibular soft tissue masses, in order to carefully plan biopsy and reconstructive surgery.