Purpose : In order to obtain relevant information for optimizing the performance of air mattresses for elderly individuals, the study was designed to, 1) Establish the relationship between the internal air pressure of the mattress and interface pressure while the subjects were at different positions. 2) To see whether healthy young individuals could serve as valid controls for this kind of evaluation. Subjects : The subjects consisted of 8 elderly bed-bound individuals with a history of pressure ulcer and 8 healthy females agod 20 years. The interface pressure was measured by a contact pressure distribution measurement device mads by NEC-Sanei Co. with subjects in either a supine or lateral recumbent position and the head of the Gatch-bed raised to 30,45 or 70 degrees. The maximum interface pressure at the internal pressure setting of 18 or 36 mmHg was compared to the known pressure level associated with ulcer formation. This was performed while the subject was lying in a range of different positions. The cellular type alternating pressure mattress was used in the study. Results : 1) When the Gatch-bed was set at any angle reduction of internal air pressure effectively achieved more uniform pressure distribution at the tissue interface for the elderly, although Gatch-bed angles with a head elevation greater than 45 degrees were associated with the risk of bottoming out of the mattress under the buttocks (loss of suspension under the buttocks). 2) In healthy subjects, changes in internal air pressure had a minimal influence on interface pressure distribution or on pressure localization over the bony prominences. This contrasted with elderly individuals in whom lower internal pressures caused a loss of suspension of their buttocks and greater internal pressures compromised their interface pressure relief capability. The study revealed these characteristics of air mattresses unique to the elderly population, clearly differentiating them from young healthy subjects, particulary with their maximum interface pressure measurements exceeding 32 mmHg, the level associated with a high risk of pressure ulcer development.