The present study was carried out on eighty patients attending Zagazig University Hospitals. Forty cases suffered idiopathic cardiac diseases (28 with cardiomyopathy, 8 with myocarditis & 4 with valvular lesions) and forty cases suffered idiopathic rheumatic diseases (30 with musculoskeletal complaints and 10 with myositis). Sera were investigated by enzyme-linked immunosorbent assay (ELISA) and indirect fluorescent antibody technique (IFAT) using Sarcocystis fusiformis antigen in order to detect the role of Sarcocystis in initiation of these diseases. Twenty positive toxoplasmic sera and sera from twenty normal individuals were considered as control group. The sera of the investigated cases were tested against Toxoplasma gondii antigen to exclude it as one of the causative agents of these idiopathic lesions. No statistical difference was found between IFAT and ELISA in diagnosis of sarcocystosis (P < 0.05). Also, there was no cross reaction between Sarcocystis and Toxoplasma. This study showed that Sarcocystis can be considered as one of the possible causes of some idiopathic diseases.