ObjectivesTo investigate the prevalence and clinical significance of anti-calreticulin autoantibodies (anti-CRT Ab) in a large cohort of idiopathic inflammatory myopathy (IIM) patients. MethodsSera from 469 patients with IIM, 196 patients with other connective tissue diseases, 28 patients with solid tumors and 81 healthy controls were screened for anti-CRT Ab by enzyme-linked immunosorbent assay using human recombinant CRT protein. Sera from 35 IIM patients were tested using an immunoprecipitation assay to confirm the presence of anti-CRT Ab. Subsequently, IIM-cancer patients were identified and divided into new-onset, remission and recurrent groups based on their cancer status. The relationships between anti-CRT Ab levels and IIM disease activity were also investigated. ResultsSerum anti-CRT Ab was detected positive in 81 of the 469 (17.3%) IIM patients. Immunoprecipitated bands were observed at a molecular weight of 60 kDa corresponding to the CRT protein. The IIM patients with anti-CRT Ab more frequently had cancers compared to the patients without anti-CRT Ab. Moreover, the prevalence of anti-CRT Ab differed according to the cancer status. The IIM patients with recurrent cancers had a much higher prevalence of anti-CRT Ab than those with cancers in remission. Also, serum anti-CRT Ab levels positively correlated with disease activity at baseline and at follow-up visits. ConclusionWe report the existence of serum anti-CRT Ab in IIM patients and demonstrate the possible association of anti-CRT Ab with malignancy in IIM patients. Serum anti-CRT Ab could serve as a novel candidate marker of cancer in IIM patients.