Abstract: Bullous pemphigoid (BP) is an autoimmune blistering disease that commonly affects elderly patients. Direct immunofluorescence (DIF) for immunoglobulin G (IgG) and C3c on frozen skin biopsies is the gold standard for the diagnosis of BP. In a minority of cases, IgG and/or C3c are found negative, and in these situations, there is a need for a more stable diagnostic marker of BP. C4d is biologically inactive, but has a long half-life, rendering it a long-lived marker for antibody-mediated complement activation. Previous studies already demonstrated that C4d was diagnostically useful in formalin-fixed paraffin-embedded skin biopsies of patients with BP. We hypothesized that C4d detected by DIF could also be a promising diagnostic marker for BP, particularly in IgG and/or C3c DIF-negative cases. In this single-center retrospective study, 69 cases of BP were analyzed for linear deposition of C4d; of the 69 cases, n = 26 were IgG+/C3c-, n = 10 IgG+/C3c+, and n = 33 IgG-/C3c-. Results were compared with n = 39 negative controls. Seven of the 26 (27%) IgG+/C3c- and 3 of the 33 (9%) IgG-/C3c- BP cases were positive for C4d. All 10 IgG+/C3c+ cases were also C4d positive. In the negative control group, 2 of the 39 (5%) were found positive for C4d. In conclusion, the current study shows that C4d is a more sensitive but not a 100% specific marker of BP. We conclude that C4d by DIF could be an interesting diagnostic adjunct for BP, particularly in IgG-/C3c- double negative cases., Competing Interests: B. Horvath reports fees from Janssen-Cilag (advisory boards, educational grants, consultations, and investigator initiative studies), AbbVie (advisory boards, educational grants, consultations, and investigator initiative studies), Novartis Pharma (advisory boards, consultations, and investigator initiative studies), UCB Pharma (advisory boards and consultations), Leo Pharma (consultations), Solenne B. V. (investigator initiative studies), Celgene (consultations and investigator initiative studies), Akari therapeutics (consultations and investigator initiative studies), Philips (consultation), Roche (consultation), Regeneron (consultation), and Sanofi (consultation), fees which were paid to the institution. The other authors declare no conflicts of interest., (Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.)