The 30 degree laterally inclined and 30 degree head elevated positions (hereafter referred as the 'rule of 30' unless otherwise specified) are widely used as a means of both primary and secondary prevention of pressure ulcers as a result of reductions in localised pressures over bony prominences. However, the authors observed that some some parts of the wound margin were thickened. These thickened edges may be caused by use of the rule of 30 positioning and may also be responsible for a delay in the healing process. This study included five bedbound elderly patients with pressure ulcers located at the sacrum and coccyx. The local pressure was measured at the thickened edges and normal edges of the subjects' wounds by a newly developed sensor while the subjects were positioned according to the rule of 30. The results showed the maximum pressure as well as the average pressure of the thickened edges to be significantly greater than that of the normal edges. Thus, it is suggested that higher pressure on different areas of the wound margin may be responsible for the thickened edges phenomenon, which may consequently delay the healing process. Clinical use of the rule of 30 for patients with pressure ulcers in the sacrum and coccyx regions should be reconsidered.