In this research, LV systolic functions of the 14 mature dogs with idiopathic pericardial effusion (IHPE) and cardiac neoplaesia (CN) were examined. The dogs were between 1-16 years old, generally large breed and male. Signs of cardiomegaly were observed in the chest radiography and low amplitude QRS complexes in electrocardiograms. Pericardial effusion, cardiac masses and paradoxical motion were detected by 2D echocardiography. Left ventricle (LV) systolic functions were detected with M mode echocardiographic examinations. A statistical significance (p<0.001) was found between echo-free pericardial Space (EPS) and fractional shortening (FS%), ejection fraction (EF%), velocity of circumferential fiber shortening (VcF), in the dogs with IHPE. A distinction was also found between EPS and EF in the dogs with CN (p < 0.001). FS, EF and VcF were significantly decreased in the dogs with IHPE in contrast to the dogs with CN. It was detected that the systolic function of the left ventricle could be considerably disturbed by the excessive volume of the pericardial effusion, even if no cardiac tamponade occurred.