BACKGROUND AND OBJECTIVES: The differential diagnosis of a regular tachycardia, with a wide QRS complex(> or =20 msec) n the 12-lead electrocardiogram(CG), remains an important challenge. Despite the information available on electrocardiography in patients with ventricular tachycardia(T) nd supraventricular ventricular tachycardia, with aberrant conduction or pre-existed bundle branch block(ide-QRS SVT); the data from Korean is limited. The purpose of this study was to report on the differential diagnostic criteria.SUBJECTS AND METHODS: The clinical and electrocardiographic characteristics of 150 patients(5.8%) ith VT, and 78(4.2%) ith wide-QRS SVT, were evaluated. RESULTS: n atrioventricular(V) issociation was found in 44.7%(7/150) f the VTs, which is very useful in differential diagnosis. In the RBBB patterns, a monophasic R, or a biphasic, wave(R, RS), in the V1 lead, were present in 49.0 and 40.6% of VTs, respectively, and an R/S ratio of less than 1, in the V6 lead, was present in 89.6% of VTs. In the LBBB patterns, the duration of the R wave(gt; or =0 msec), the interval from the Q wave to the nadir of the S wave(> or =0 msec) nd the notching of the S wave, in the V1 lead, were present in 61.1, 87.0 and 31.3% of VTs, respectively. When an algorithm, using the AV dissociation and morphological criteria for a VT in the V1 and V6 leads, was tested for differentiation, the sensitivity and specificity were 96.7 and 93.6%, respectively. CONCLUSION: A more accurate, correct, diagnosis of wide-QRS tachycardia can be made by using a stepwise approaching method consisting of AV dissociation and morphological criteria for VT in the V1 and V6 leads.