A comparative study of the ultrastructure of two cases of ecchordosis physaliphora and of two chordomas demonstrated outstanding similarities in the cells composing these two entities. Chordoma cells possessed a more prominent Golgi apparatus, an endoplasmic reticulum-mitochondria complex, plasmalemmal infoldings, nuclear irregularities and a larger extracellular space with abundant matrix, reflecting their expanding neoplastic nature. Ecchordosis cells had nuclear inclusions, dense-core membrane-bound granules and subcytoplasmalemmal linear densities, not previously described. Both ecchordosis and chordoma cells have morphologic features of both epithelial and mesodermal character. This study supports the concept that chordomas arise from heterotopic notochordal remnants in the cranio-vertebral canal.