This paper reports a systematic review conducted to determine the effectiveness of the use of risk assessment scales for pressure ulcer prevention in clinical practice, degree of validation of risk assessment scales, and effectiveness of risk assessment scales as indicators of risk of developing a pressure ulcer. Pressure ulcers are an important health problem. The best strategy to avoid them is prevention. There are several risk assessment scales for pressure ulcer prevention which complement nurses' clinical judgement. However, some of these have not undergone proper validation. A systematic bibliographical review was conducted, based on a search of 14 databases in four languages using the keywords pressure ulcer or pressure sore or decubitus ulcer and risk assessment. Reports of clinical trials or prospective studies of validation were included in the review. Thirty-three studies were included in the review, three on clinical effectiveness and the rest on scale validation. There is no decrease in pressure ulcer incidence was found which might be attributed to use of an assessment scale. However, the use of scales increases the intensity and effectiveness of prevention interventions. The Braden Scale shows optimal validation and the best sensitivity/specificity balance (57.1%/67.5%, respectively); its score is a good pressure ulcer risk predictor (odds ratio = 4.08, CI 95% = 2.56-6.48). The Norton Scale has reasonable scores for sensitivity (46.8%), specificity (61.8%) and risk prediction (OR = 2.16, CI 95% = 1.03-4.54). The Waterlow Scale offers a high sensitivity score (82.4%), but low specificity (27.4%); with a good risk prediction score (OR = 2.05, CI 95% = 1.11-3.76). Nurses' clinical judgement (only considered in three studies) gives moderate scores for sensitivity (50.6%) and specificity (60.1%), but is not a good pressure ulcer risk predictor (OR = 1.69, CI 95% = 0.76-3.75). There is no evidence that the use of risk assessment scales decreases pressure ulcer incidence. The Braden Scale offers the best balance between sensitivity and specificity and the best risk estimate. Both the Braden and Norton Scales are more accurate than nurses' clinical judgement in predicting pressure ulcer risk.