A large number of clinical measures of psoriasis are used in clinical trials and daily practice. These measures lack uniformity and validation. However, valid outcome and severity measures for psoriasis are a prerequisite for fully informative clinical research and evidence-based medicine. The purpose of this study was to identify all clinical measures of psoriasis severity and outcome in use and to evaluate the quality of these measures using clinimetric criteria; we identified 53 separate clinical measures, which were regrouped into 11 measures for quality analysis. No measure could be scored on all items used in the clinimetric analysis. The Lattice System Physician's Global Assessment and Physician's Global Assessment were most highly noted. We conclude that none of the psoriasis measures is adequately validated. The Psoriasis Area and Severity Index is the most commonly used clinical measure in research, but it has substantial limitations such as low response distribution, no consensus on interpretability, and low responsiveness in mild disease. Nevertheless, because of its widespread use the Psoriasis Area and Severity Index permits some degree of comparison of results among clinical trials. Overall, no best instrument was identified, and different situations may call for different measures.