The differences in the posterior arch widths between opposing maxillae and mandibles were studied in 100 edentulous denture-wearing patients. Panoramic radiographs of 67 of these patients were used to assess the percentage of mandibular height reduction from that considered present before extractions. Comparisons between posterior maxillary and mandibular arch widths, reductions in mandibular height, and edentulous age accompanied by denture wearing were made. The following conclusions emerged: 1. The mandible does not progressively widen as its opposing maxillae becomes narrower, nor does a progressive incremental increase occur in posterior maxillomandibular width relations with increased resorption or resorptive age. 2. The arch width of the mandible exceeds the arch width of the maxillae in the molar region by an average of 6 to 7 mm after sufficient resorption establisheds a definitive alveolar crest. This difference remains the same regardless of the amount of resorption or resorptive age. 3. The progressive and irreversible mandibular alveolar resorption rate is greatest in the earlier stages of edentulism and slows with loss of bone, longevity of edentulism, and attendant wearing of dentures.