Neurofibromatosis type 1 (NF1) is a common dominantly inherited disease. More than half of NF1 patients suffer from skeletal manifestations, of which congenital pseudarthrosis of tibia (CPT) is one of the most incapacitating lesions. Two NF1 patients with CPT were operated, and the resected tissues were analyzed using immunohistochemistry and/or in situ hybridization for NF1 protein and mRNA, p-p44/42 MAPK, and S100 protein. Both patients displayed thick-walled arteries and veins with a small lumen within the fibrotic tissue in the vicinity of pseudarthrosis. Endothelial cells were highly positive for p-p44/42 MAPK. A subpopulation of cells surrounding the blood vessels was S100 protein-positive. However, the exact identity of the S100-positive cells remains to be elucidated. Neurofibromin mRNA and protein labeling was detected in both cell types. In conclusion, decreased NF1 function as a RAS-GAP in the endothelium may contribute to vascular thickening in CPT.