OBJECTIVES: To validate a software, for an optical pencil toll, to calculate psoriasis patients' Body Surface Area (BSA) and to demonstrate that the new developed method is been valid and reliable to quantify the BSA. METHODS: Multicentre prospective study at Dermatology centres (Five Spanish Hospitals and one Portuguese Hospital). In each hospital two dermatologists visited the same patients twice (second visit 3 days after first). 60 dermatologists included £10 consecutive patients with psoriasis. Sociodemographical and clinical variables (PASI, time since diagnosis, current treatment) and BSA scores were colleted for each patient in the two visits. To calculate BSA scores traditional method (visual grading following the nine rule of Wallace method's) and the optical pencil method (BSA software developed) were used. Inter-intraobservers reliability, variability between BSA scores regarding the new tool, versus the traditional method, and the tool's usefulness will be assessed. RESULTS: Fifty-six patients were included. Mean (SD) age was 48.93 (16.76) years. Mean (SD) time since diagnosis was 18.77 (14.28). Pearson's correlation coefficient between both methods was 0.91 (p<0.01). Intraobserver correlation for each of the methods was 0.91. The correlation among both methods was in the first visit 0.92 and 0.90 in the second visit. The ICC was higher than 0.85 independently of which of the two methods were used firstly. The investigators considered that the new method is easy to use (94%), it guides towards the disease management (64%) and standardizes the calculation of the body surface area (86,4%). CONCLUSION: These results can prove that the software to assess BSA has shown to be valid to be used both in clinical practice and in clinical studies. Therefore, the optical pencil method to quantify BSA can be used as standard for the assessment of involved body surface area in the management of psoriatic patients.