Background Lichen sclerosus is a common acquired inflammatory disorder of skin and mucous membranes. The aetiology is unknown, although HLA-subtype susceptibility and high rates of other autoimmune disorders suggest that autoantibodies to specific mucocutaneous antigens are involved. The clinicopathological similarities between lichen sclerosus and lipoid proteinosis, which results from mutations in extracellular matrix protein 1 (ECM1), suggest this protein as an autoantigen. Methods We analysed serum autoantibody profiles in 171 individuals (86 with lichen sclerosus, 85 healthy controls) by immunoblotting of extracts from normal human skin and lipoid proteinosis skin (lacking ECM1). We generated a fulllength glutathione-S-transferase fusion protein for ECM1 to confirm specific immunoreactivity. We affinity-purified serum from patients with lichen sclerosus and did indirect immunofluorescence microscopy on normal skin with or without preabsorption with recombinant ECM1. Findings By immunoblotting, IgG autoantibodies were found in 20 (67% [95% CI 45–84]) of 30 lichen sclerosus serum samples. The highest titre was 1 in 20. The bands were not detected in ECM1-deficient substrate. These samples, and those from 56 other patients with lichen sclerosus, showed immunoreactivity to the recombinant ECM1 protein (64 of 86 positive; 74% [65–84]). Only six (7% [2–13]) of 85 control serum samples were positive. Affinity-purified IgG from serum of patients with lichen sclerosus labelled skin similarly to a polyclonal antibody to ECM1. The positive staining was blocked by preabsorption with excess recombinant ECM1 protein. Interpretation These findings provide evidence for a specific humoral immune response to ECM1 in lichen sclerosus and offer insight into disease diagnosis, monitoring, and approaches to treatment.