Juvenile aggressive fibromatosis is an acquired disease affecting young children. There are two types: superficial and deep; the first is not aggressive whilst the second invades other tissues deeply. This is a case report of the deep variant of juvenile aggressive fibromatosis of the lateral mandible affecting a 24-month-old young female patient. The tumour has been treated surgically by resection of the mandible and reconstruction with a rib-graft. To by-passs resorption of the rib-graft and to re-establish the correct three-dimensional shape of the facial skeleton, osteodistraction of the reconstructed mandible was performed six months post-peratively. In this article the surgical techniques to reconstruct the mandible in young children are discussed.