In recent few years, many authors had discussed the possibilities of using bipolar precordial leads to replace lead V5 in diagnosing anterior wall myocardial ischemia. But there still remains some problems to be solved: (1) Are there any difference between bipolar precordial leads and lead V5 in wave forms? (2) What kind of factors influence the ST deviation? 36 patients were studied by comparing unipolar lead V5 with respect to three different bipolar chest leads CM5, CC5, CL5 both in lying and sitting positions. We found that: (1) The differences is least significant between bipolar CC5 and lead V5 while it is most significant between bipolar lead CM5 and lead V5. (2) The deviation of ST segment in bipolar leads is affected both by potential at negative electrode as well as potential at positive electrode. Statistical analysis revealed that ST segment is positively related to the potential on positive electrode and negatively related to the potential on negative electrode. For this reason, therefore, in the view point of most comparable ST deviation with lead V5 for monitoring of myocardial ischemia, it is advisable to locate the negative electrode at the site on chest with least potential. Usually we place the negative electrode on V6R to compose a bipolar lead CC5.