The rarity of relapsing polychondritis (RP) has hindered the development of standardized tools for clinical assessment. Here, we describe the development of a preliminary score for disease assessing activity in RP, the Relapsing Polychondritis Disease Activity Index (RPDAI).Twenty-seven RP experts participated in an international collaboration. Selection and definition of items for disease activity were established by consensus during a 4-round internet-based Delphi survey. Twenty-six experts assessed the Physician's Global Assessment (PGA) of disease activity on 43 test cases on a 0-100 scale, yielding a total of 1118 PGA ratings. The weight of each item was estimated by multivariate regression models with generalized estimating equation, using PGA as the dependent variable.Experts decided in consensus that the RPDAI should consider the 28-day period before each RPDAI assessment. Inter-rater reliability assessed by the intra-class correlation coefficient for the 1118 PGA ratings was 0.51 (CI95%: 0.41-0.64). The final RPDAI score comprised 27 items with individual weights ranging from 1 to 24 and a maximum theoretical RPDAI score of 265. Correlation between the RPDAI scores calculated based on the weights derived from the final multivariate model, and the 1118 PGA ratings was good (r=0.56, p<0.0001).We have developed the first consensus scoring system to measure disease activity in relapsing polychondritis (see www.RPDAI.org for online scoring). This tool will be valuable for improving the care of patients with this rare disease.