OBJECTIVE: To use a simulated upper jaw made from a rubber arch containing replaceable ceramic teeth and a renewable composite ceramic jawbone to compare the effectiveness of seven custom made mouthguard designs and a "boil and bite" mouthguard. METHODS: Following an earlier development of a standard impact test using a selection of projectile shapes and energies, the most sensitive conditions were selected. These were then applied to a series of six guards constructed in ethylene vinyl acetate and styrene butadiene. The guards were constructed to reflect possible variations in both design and materials. RESULTS: Significant differences between the mouthguard performances were observed in response to the impact conditions selected. All the custom made designs gave better performance than the "boil and bite" mouthguard. CONCLUSIONS: The differences observed indicated that the standard test should be sensitive enough to be used as an assessment procedure for the approval of the manufacture of these safety devices. A sequence of tests on eight identical mouthguards selected from a batch of 12, based on the best design, gave remarkably consistent results, indicating that both the manufacturing technique and the test method are reliable. The features of the best design should be incorporated into the current "best practice" for the construction of these devices.