Three young children with characteristic fibroblastic tumors are presented. These comprised one case each of congenital fibrosarcoma, dermatofibrosarcoma protuberans, and fibrous hamartoma of infancy. Ultrastructural examination in each tumor revealed the presence of a peculiar intracisternal paracrystalline material that exhibited transverse periodicity between 18 and 22 nm. Elongated profiles lying within rough endoplasmic reticulum had a serpigenous outline. It is thought that this material is related to procollagen or its precursors and its presence may signify disorders of fibroblastic proliferation that may occur in fibrous tumors.