Pressure ulcers are a serious problem affecting over a million patients every year. Despite accepted guidelines for assessing and repositioning high-risk patients, the prevalence of pressure ulcers continues to rise. This paper presents a wearable, wireless vital sign monitor capable of continuously measuring the duration and orientation of a patient's posture throughout the patient's stay in a hospital. Patient posture is determined using a tri-axial accelerometer attached to a patient's torso. A novel set of algorithms are used to process the accelerometer signals to adaptively identify accelerometer alignment on the patient, calculate patient spine angle, and classify patient orientation. A unique pressure ulcer risk index based on these variables is presented to assess a patient's risk for developing pressure ulcers. Experimental results from an 18 subject trial are also presented.