The prevalence, clinical associations and pathogenic role of newly identified autoantibodies to the erythropoietin receptor (EPOR) in patients with anaemia were investigated. Sera from 203 patients with immune-related or chronic kidney diseases were screened for anti-EPOR antibodies by enzyme-linked immunosorbent assay, and antibody specificity was evaluated by immunoprecipitating EPOR from AS-E2 cells using purified immunoglobulin (Ig) fractions. In addition, the pathogenic role of anti-EPOR antibodies was determined by examining their inhibitory effects on AS-E2 cell proliferation. Clinical findings were compared between patients with and without anti-EPOR antibodies, in all patients and those with systemic lupus erythematosus (SLE). Serum anti-EPOR antibodies were detected in 52 patients. Purified IgG or IgM fractions from anti-EPOR antibody-positive sera immunoprecipitated EPOR and inhibited the EPO-dependent proliferation of AS-E2 cells in a dose-dependent manner. Anti-EPOR antibodies were associated with low haemoglobin concentrations and reticulocytopenia in all patients enrolled and those with SLE. Further, there was a negative correlation between the levels of anti-EPOR antibodies and the number of bone marrow erythroblasts in patients who underwent bone marrow examinations. These findings suggest that EPOR autoantibodies are present in a subset of patients with anaemia and that impaired erythropoiesis can be mediated by anti-EPOR antibodies, which functionally neutralize EPO activity.