Purpose: Forceful clenching in the intercuspal position has the potential to cause significant mandibular displacement. Such a displacement can be expected to be exaggerated in patients without molar support. The appropriate clenching level for intercuspal position registration or evaluation in these patients has never been clarified. The aim of this study was to clarify the effect of clenching level and absence of posterior occlusal support on mandibular displacement. Materials and Methods: Thirteen women with a unilateral edentulous area posterior to the first premolar were enrolled in this study. They were asked to perform intercuspal clenching at various occlusal force levels, and 3-D mandibular displacements were measured by an optoelectronic method. The effects of ¡°clenching force level¡± and ¡°side of molar tooth loss¡± on the displacements of the condylar and second molar points were analyzed. Results: The mandible was elevated without substantial horizontal displacements during each clenching task. The effect of clenching level on vertical mandibular displacement was significant, with a clenching level ¡İ 50% of maximal voluntary contraction causing significantly greater elevations (P < .05). Clenching ¡İ 50% of maximal voluntary contraction caused consistent tipping of the mandible, with greater elevation on the edentulous side (P < .05). Conclusion: The clenching level should not surpass 50% of maximal voluntary contraction for registration or evaluation of intercuspal position in Kennedy Class II patients. Int J Prosthodont 2003;16:183¨C188.