Histologic analysis of 22 cases of fibrous dysplasia was performed. 1. In each case separately 100 immature bony trabeculae were counted, giving a total of 2200 counts. It was concluded, that on a given section, 75,7 per cent of the trabeculae were in contact with vascular cavities. The remaining 24,3 per cent seemingly "metaplastic" bony trabeculae were examined further by serial section. In different layers of the block material the obvious connection with vascular spaces could be shows. 2. The vascular (not metaplastic) origin of immature bony trabeculae was demonstrated too. The development of this pattern of bone starts and continues in contrast to normal ossification within the capillaries and sinusoids in an "inverted" manner. Later the trabeculae "grow out" from the vascular cavities but keep their contact with them.