Changes in the vertical dimension of mandibular rest position for 50 subjects wearing complete dentures were measured on cephalometric radiographs between lead markers attached to the face and between skeletal landmarks. Measurements between the landmarks were compared on radiographs made without (edentulous) and with (edentoprosthetic) the dentures in the mouth. The findings support the following statements. The facial structures tended to mask the positional changes of the mandible. The rest position of the edentulous mandible tended to vary within the time frame of the cephalometric examination. The range of measurements of the skeletal rest position appeared greater with greater denture age and experience of the subjects. The rest position of the edentoprosthetic mandible (with dentures in the mouth) was more frequently constant when measured skeletally than when measured facially and more frequently constant than the edentulous mandibular rest position. The edentulous mandibular rest position of most subjects was increased upon insertion of dentures. The rest position was decreased for the remainder of the subjects. As a group, those subjects whose mandibular rest position decreased (up-shifted) with insertion of dentures had older dentures and more years of denture-wearing experience than subjects whose mandibular rest position increased (down-shifted) with insertion of dentures. The edentulous mandibular rest position does not seem particularly suitable for determination of the vertical dimension of occlusion and appears somewhat less suitable when facial measurements are used.