Congenital pseudarthrosis of the leg remains one of the most controversial pediatric entities in terms of etiopathogenesis, pathology, treatment, and prognosis. The authors reviewed the pathologic material of 24 patients with congenital pseudarthrosis of the leg along with clinical and radiographic data. The tibia was affected in 22 patients; in two patients the disease was limited to the fibula. Fifteen patients were male and nine were female. Age at surgery ranged from 1 to 26 years. Nineteen patients were classified as having dysplastic type, one cystic, and four mixed. Clinical evidence of neurofibromatosis type I (NF-I) was found in 17 patients. The main histopathologic change observed was the growth of a highly cellular, fibromatosis-like tissue. In the dysplastic type, such tissue was associated with the periosteum. In the cystic type, a closely similar tissue occupied the lytic area. In case classified as of mixed type, the coexistence of endosteal/medullary and periosteal involvement by the fibromatosis-like tissue was observed. In the cystic lesion, evidence of de novo bone formation within the lesional tissue was obvious. Overall, the histologic features of the cystic lesion were similar to those of osteofibrous dysplasia. In the dysplastic type, the proliferation of the fibrovascular tissue was associated with active osteoclastic resorption of the cortex, which remodeled into a trabecular rather than a compact type of structure. Histologic comparison of the pathologic samples of patients with and without NF-I revealed no significant differences. The pseudarthrosis gap was continuous with periosteal soft tissues and filled by fibrous tissue, fibrocartilage, and hyaline cartilage with features of enchondral ossification. The authors suggest that the clinical diversity of congenital pseudarthrosis of the leg results from the diverse location of a single pathologic process--namely the growth of an abnormal, fibromatosis-like tissue either within the periosteum or within the endosteal/marrow tissues. It is tempting to suggest that such an "osteofibromatosis" represent a skeletal expression of neurofibromatosis, either within the fully expressed syndrome (patients with known neurofibromatosis) or as isolated lesion (patients with unknown/cryptic neurofibromatosis).