It is often helpful to assess the pressures exerted upon the bony prominences when monitoring the likely outcome of pressure ulcer prevention or treatment. However, in the clinical setting, hard pressure sensors may damage the skin and operational difficulties may influence their reliability and validity. The authors have developed a multi-pad pressure sensor and tested its clinical reliability and validity. The inter-rater and intra-rater reliability were calculated using the coefficient of variation data from 10 patients. After a comparison analysis, the multi-pad was more reliable than a single-pad type pressure sensor. A validation test was conducted in 79 elderly patients. The mean interface pressures recorded among patients who had erythema or stage I pressure ulcers at the sacrum were significantly higher than were the contact pressures measured in patients with no pressure damage. The pressure sensor exhibited satisfactory clinical reliability and validity. Furthermore, it may be that for Japanese elderly patients the maximum pressure that can be tolerated by the tissues around the sacrum may be 40-50 mmHg.