Purpose: The purpose of this study was to evaluate denture base displacement caused by tongue movements during speech. Materials and Methods: Ten patients with new Kennedy Class I mandibular removable partial dentures participated in this study. All prostheses were made according to a standardized protocol. Denture base displacements when speaking a test sentence were measured with a computer-aided device that was developed for this purpose. It included 4 Hall sensors, mounted on a paraocclusal splint and magnets, embedded in the denture bases. Vertical and transverse base displacements were calculated from Hall voltages. Clasp retention was measured with a dynamometer. Results: Mean displacements were 145 um seating, 72 um unseating, 87 um buccally, and 74 um lingually. Seating and unseating displacements differed statistically significantly in absolute values. Mean clasp retention was 2.5 N. No correlation was found between clasp retention and unseating base displacements. Conclusion: Tongue movements during speech induced only minimal displacements in the dentures investigated. Additionally, tongue activity was found to be denture stabilizing rather than denture displacing in this study.