The Bullous-Pemphigoid-Disease-Area-Index (BPDAI) score has been proposed to provide an objective measure of bullous pemphigoid (BP) activity.To calculate BPDAI cut-off values defining mild, moderate and severe BP, and to assess the inter-rater reliability, and the correlation with the number of daily new blisters, and anti-BP180 and anti-BP230 antibodies.Severity scores were recorded by two blinded investigators. Anti-BP180 and anti-BP230 antibodies were measured using an ELISA assay. Cut-off values defining mild, moderate and severe subgroups were calculated based on the 25th and 75th percentiles of the BPDAI score.Two-hundred-eighty-five BP patients were enrolled in 50 dermatology departments in Europe. Median BPDAI activity was 37.5 points [range=0-164]. Cut-off values corresponding to the first and third quartiles of the BPDAI score were 20 and 57, thus defining mild (≤ 19), moderate (≥ 20 and ≤ 56), and severe (≥ 57) BP. The median BPDAI score of patients with ≤10 daily new blisters was 26 (IQR=17-45 points), and that of patients with > 10 daily new blisters was 55 (IQR= 39-82 points). The BPDAI intraclass correlation coefficient measured at baseline was 0.97 and remained higher than 0.90 up to Month 6. The improvement of the BPDAI score was correlated with the absolute decrease of anti-BP180 ELISA value (Spearman's rank, r=0.34, p<0.0037), but not with anti-BP230 antibodies (r=0.17, p=0.15).This study suggests cut-off values of 20-57 for BPDAI to distinguish mild, moderate and severe BP, and confirms that it is a robust tool to precisely assess BP severity.