The 3-dimensional forces exerted on an abutment tooth of a removable partial denture (RPD) were measured in vivo during clenching using a force-measuring device with a piezoelectric transducer. The device was mounted on the mandibular right second premolar of a subject with an edentulous maxilla. The magnitude of the forces was higher and the direction was more posterior without the RPD in place. The direction was most posterior with an RPD with a distal rest only and most anterior with an RPD with a mesial rest only. The 3-dimensional forces exerted on an abutment tooth thus depend on both the presence of a denture and the rest location.