Cardiac involvement in idiopathic inflammatory myopathies (IIM) adversely affects prognosis but is commonly sub-clinical. Cardiac magnetic resonance imaging (CMR) is an effective imaging modality for detecting myocardial inflammation and fibrosis but its use as a screening tool for cardiac disease in IIM has not been fully explored. Nineteen patients with IIM without cardiac symptoms underwent CMR using a specific cardiomyopathy protocol including specific sequences detecting focal and diffuse myocardial fibrosis. 9/19 patients demonstrated late gadolinium enhancement (LGE (3/9 right ventricular insertion, 1/9 sub-endocardial, 7/9 mid-wall/sub-epicardial)). T1 mapping was performed in 15 patients. In total, 7/15 had elevated native T1 values, of which four had detected LGE. Myocardial fibrosis was frequently detected in IIM patients without cardiac history. Detection of LGE and elevated T1 values may have negative prognostic implications. Longitudinal studies determining whether early or augmented treatment has a role in patients with sub-clinical cardiac involvement are needed.Key Points• Cardiac involvement in myositis adversely affects prognosis.• Cardiac magnetic resonance imaging is an effective tool for detecting cardiac involvement.• T1 mapping is a technique which detects diffuse myocardial inflammation and fibrosis.• In our study, focal and diffuse myocardial fibrosis was frequently found in myositis patients without cardiac symptoms.