Critically ill patients are at a particular risk for developing pressure ulcers. Yet until now, no sufficiently specific, validated pressure ulcer risk assessment instruments exist for critically ill patients. In a prospective study of 698 patients of medical intensive care unit (ICU), we therefore analyzed if the Waterlow scale is suitable for pressure ulcer risk assessment in the ICU. Only patients with no pressure ulcer on admission to the ICU were included. The Waterlow scale was used to assess pressure ulcer risk on admission to the ICU, and the number of points on the scale were analyzed with regard to pressure ulcers development in the course of the ICU stay (121 patients). Our results show that adequate pressure ulcer risk assessment on admission to the ICU is not possible with the Waterlow scale. Sensitivity and specificity reached their maximal values of 64.6% and 48.8%, respectively, at a comparably high cut-off of 30 points on the Waterlow scale (positive and negative likelihood ratio being 1.26 and 0.73, respectively). The area under the curve (AUC) was 0.59 in the receiver-operator-characteristic curve. Adding intensive care related parameters to the scale yielded some degree of improvement (AUC 0.69), but the development of ICU specific pressure ulcer risk scales still seems to be necessary to allow reliable pressure ulcer risk assessment in the ICU.