Pressure ulcer classification systems are based on the clinical manifestations of the skin and tissue layer affected rather than underlying pathology. The objective of this study was to compare the validity of the clinical grading of erythema (blanching and nonblanching) with a measurement of skin perfusion. Therefore, an exploratory study comparing erythema with laser Doppler imaging of the sacrum and buttock skin areas was undertaken. Acute and major elective general, vascular, and orthopedic surgical in-patients, aged 55 years or over with an expected length of hospital stay of 5 or more days were recruited. Fifty laser Doppler images from 37 patients were obtained and included in a discriminant analysis. Discriminant analysis suggested that blanching and nonblanching erythema were physiologically distinct from "normal" skin; clinically, these could be assessed with reasonable accuracy. Imaging also determined that high blood flow of differing intensity characterized blanching and nonblanching erythema. There was no evidence of the "no flow" phenomenon.