Fibro-osseous lesions involving the paranasal sinuses, the mid-face and anterior skull base are uncommon. In addition, there appears to be no clear pathological or clinical classification that embraces the variety of lesions that exhibit such diverse pathological and clinical behaviour, yet may still be referred to as a fibro-osseous lesion. The diagnosis of fibrous dysplasia and ossifying fibroma is made on a combination of clinical, radiological and pathological criteria. This paper emphasizes the clinical and pathological differences between fibrous dysplasia and ossifying fibroma. The more aggressive clinical behaviour of the latter is highlighted and a more radical surgical approach is recommended. In contradistinction, fibrous dysplasia can exhibit a more benign behaviour and radical surgery is not always justified. A clinicopathological distinction between these two conditions is important from a management perspective despite the fact that they both may be encompassed under the 'umbrella' term fibro-osseous lesion.