Regarding incidence of PERICARDIAL EFFUSION and its side effects, ethiology of this disease were studied in this research. Also cost of pericardiocentesis in diagnosing PERICARDIAL EFFUSION, which is an invasive method, was evaluated.100 patients with PERICARDIAL EFFUSION admitted in heart department of hospital were the subjects of our study. In information, including clinical finding, preclinical examination, echocardiography and perdicardiocentesis were gathered by means of a question form for each patient.Idiopathic, TB, Radiation, Cancer, uremia, trauma, post operation and hypothyroidism ordered by importance were found as primary reasons for incidence of PERICARDIAL EFFUSION.According to our statistical tests, ethiology of PERICARDIAL EFFUSION and pericardiocentesis, ethiology of PERICARDIAL EFFUSION and sex, pericardiocentesis and sex are related consequently.Also, based on these results it seems that pericardiocentesis is a cost-effective diagnostic method.