Fabry disease is a lysosomal storage disorder with onset of adverse signs and symptoms usually during childhood and progressive life-threatening decline in organ functions. A validated and feasible Fabry disease severity scoring system (DS3) is needed to reliably quantify the disease burden, monitor disease progression and treatment response, and compare disease status among patient cohorts in clinical studies. We developed a new Fabry DS3 and tested its reliability and validity using a combination of expert consensus formation and statistical techniques. Relevant Fabry disease domains and items were identified, inclusion of items was refined and scaling of scores for individual assessments was optimized to maximize the correlation between the instrument's total score and the assigned clinical global impression of severity (CGI-S scores). Furthermore, the minimum clinically important difference in each of the instrument's domains was estimated and the DS3's quantitative content validity was judged. The current Fabry DS3 working model has 5 domains; 4 clinical domains (Peripheral Nervous System, Renal, and Cardiac, each with 3 items, Central Nervous System with 2 items) and a patient-reported domain (Patient-Reported domain with one item). The domain score is obtained by averaging the scores for all domain items. The Content Validity Index and Feasibility Index were shown to be good; 0.96 and 0.97, respectively. There was no significant inter-rater difference and the level of concordance was high. Correlation with the CGI-S was R2 = 0.89 indicating excellent criterion and construct (convergent) validity. In summary, initial estimations of validity, reliability and feasibility for the new Fabry DS3 instrument suggest that it is a feasible and reliable means of assessing disease severity and progression over time and comparing inter-patient severity of Fabry disease. Our results demonstrate that the Fabry DS3 correlates highly with the clinical assessment by Fabry disease experts. © 2009 Elsevier Inc. All rights reserved.