The resistance to fracture of jacket crowns, fabricated of cast glass-ceramic and five traditional ceramics, was evaluated. Four identical jacket crowns of each type were constructed on absolutely equal dental abutments in epoxy resin, obtained from a primary mandibular molar. An increasing load was then applied to the mesiobuccal cusp of each of the 24 crowns. Although the glass-ceramic proved to have the highest fracture resistance, it was not significantly stronger than three other traditional ceramics; neither did it seem to be suitable for constructing jacket crowns on posterior teeth.