Pemphigoid diseases are a heterogeneous group of autoimmune blistering disorders characterized by predominant deposition of immunoglobulin G or immunoglobulin A autoantibodies against structural proteins of the dermoepidermal junction (DEJ). Sole linear immunoglobulin M (IgM) deposits at the DEJ in pemphigoid diseases have been observed; however, IgM-specific target antigens have not been identified.Characterization of patients with IgM pemphigoid.Skin biopsy specimens and sera from IgM-positive patients were assessed using histopathology, direct and indirect immunofluorescence microscopy, enzyme-linked immunosorbent assays, immunoblotting, cryosection assay, complement fixation test, and internalization assays.Tissue-bound linear IgM deposits along the DEJ and circulating IgM autoantibodies against type XVII collagen (Col17) were detected. These circulating IgM autoantibodies showed no complement activating or blister inducing capacity, but the ability of Col17 internalization ex vivo.Limited number of patients.This study provides further evidence for the role of IgM autoantibodies in pemphigoid disease and highlights Col17 as a target antigen in IgM pemphigoid.