Since early intervention using corticosteroids improves prognosis in some patients with cardiac sarcoidosis, early and accurate diagnosis of this clinical condition is important. However, it is still not easy to evaluate the activity of cardiac sarcoidosis in clinical practice. The aim of this study was to determine whether high-sensitive cardiac troponin T (hscTnT) is useful as an additional parameter to standard assessment in patients with cardiac sarcoidosis. Twelve patients who were diagnosed as having cardiac sarcoidosis at our institution were retrospectively studied. Evaluation of patients included clinical examinations, electrocardiography, echocardiography, 67-gallium-citrate (Ga) scintigraphy, ¹⁸F-fluoro2-deoxyglucose positron emission tomography (¹⁸F-FDG PET) and laboratory data including hs-cTnT, angiotensin-converting enzyme (ACE), lysozyme and B-type natriuretic peptide (BNP). The activity of cardiac sarcoidosis was judged mainly by using ¹⁸F-FDG PET. Localized uptake of ¹⁸F-FDG, which was considered to be active cardiac sarcoidosis, was seen in 8 patients. Based on the findings of ¹⁸F-FDG PET, hs-cTnT was considered to be a reliable parameter: sensitivity and specificity were 87.5% and 75.0%, respectively. The positive predictive value (PPV) and negative predictive value (NPV) were 87.5% and 75.0%, respectively. On the other hand, these values in lysozyme and BNP markers were not as high as those in hs-cTnT. Although an ACE marker and Ga-67 scintigraphy showed specificity and PPV of 100%, both sensitivity and NPV were less than 50%. Furthermore, hs-cTnT levels decreased after steroid therapy in some patients. Hs-cTnT seems to be a useful marker for evaluating the activity of cardiac sarcoidosis.