The known facts concerning unicameral bone cyst, such as the anatomical distrbution, age incidence, histology, characteristics of the contained fluid, and the development in areas of rapid bone remodeling, lend credence to the theory of pathogenesis which postulates a sequence of events initiated by vascular obstruction in a focus of fibrous tissue located in an area of rapid remodeling of bone. The present two cases, which are the first on record which were observed during the early stage of development, may lend support to this theory since they were preceded by lesions which may very well have represented foci of fibrous tissue in non-ossifying fibromas. Since non-ossifying fibroma is a very common lesion, however, it is strange that this sequence of events has never been observed arid reported previously.