A new clinico-pathologic entity is described. It is defined as osteofibrous dysplasia of long bones, and is based on twenty two personal observations to which are added seventeen cases from the literature. This dysplasic congenital lesion is clearly differentiated from fibrous dysplasia by clinical, radiographic and histological characteristics and by its clinical course. These features may be summarised as follows: 1) Slight predominance of the male sex. 2) Very early age of onset either at birth or in the first years of life. 3) Site almost exclusively tibial, sometimes also in the fibula. Localisation predominantly in the middle third of the tibial diaphysis, but sometimes in the distal or proximal third. In the fibula, it is always at the distal third. 4) The lesion is painless and generally causes bony enlargement. There is often slight anterior bowling and more rarely, slight varus of valgus bowing. Pathological fracture may occur; rarely there is a pseudarthrosis. 5) The radiographic appearances are very characteristic, with enlargement of the bone, intracortical osteolytic lesions with thinning or disappearance of the external cortex, sclerotic reaction on the medullary aspect, and narrowing of the medullary canal. 6) The histological features are also typical, consisting of fibrous tissue enclosing bone trabeculae lined by osteoblasts and a "zonal" architectural pattern. 7) Sometimes the lesion tends to heal spontaneously in the very early years of life; in other cases it is moderatley progressive. It relapses frequently after curettage, but such recurrences are generally non-progressive. In some cases slight anterior bowing persists permanently. 8) Surgery should be restricted to patients over the age of five in whom the lesion is extensive, with imminent or actual pathological fracture, and to the rare cases of pseudarthrosis. The results are good even in cases of relapse or pseudarthrosis. The correction of residual bowing, if indicated, can safely be carried out with one or more osteotomies at the age of ten to twelve years.