Cardiac involvement is the major determinant of morbidity and mortality in patients with sarcoidosis, but clinical evaluation of the disease activity is occasionally difficult in cardiac sarcoidosis. The present study examined whether serum levels of interleukin-10 (IL-10) could reflect the disease activity of patients with cardiac sarcoidosis. Serum IL-10 levels were measured using an enzyme-linked immunosorbent assay, and compared with clinical manifestation, levels of angiotensin-converting enzyme (ACE), levels of lysozyme and accumulation of gallium-67 citrate. Sera were collected from 8 patients with cardiac sarcoidosis (CS group), 22 patients with miscellaneous heart diseases except for sarcoidosis (MHD group), and 8 healthy control subjects (HC group). Serum IL-10 levels of the CS group were significantly higher than those of the 2 control groups. Before steroid therapy, the levels of IL-10 in the CS group showed a significantly positive correlation with levels of ACE (r=0.868, p<0.05) and lysozyme (r=0.890, p<0.05). In 5 patients who were analyzed before and after steroid therapy, the levels of IL-10 tended to correlate with a decrease of an abnormal accumulation in gallium-67 citrate. Serum IL-10 levels may play a role in evaluation of the disease activity in patients with cardiac sarcoidosis.