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A 55 year old man presenting with upper abdominal discomfort and orthopnoea, and sustained wide QRS tachycardia in which the QRS configuration showed right axis deviation and right bundle branch block pattern on ECG, was referred to our hospital. The administration of intravenous lidocaine (lignocaine) (100 mg) had successfully terminated the tachycardia. The echocardiogram showed an extra chamber next to the posterolateral region of the left ventricle adjacent to the mitral annulus. The cardiac catheterisation revealed no abnormal haemodynamic findings. Left ventriculography showed a giant diverticulum, 3 × 6 cm in size, of the left ventricle, which was located posterolaterally (below left: Ao, aorta, Div, diverticulum; LV, left ventricle). The ostium of the diverticulum was opened at the posterolateral wall adjacent to the annulus of the mitral valve, and the diverticulum itself did not show active systolic contraction. In the electrophysiologic study, no ventricular tachyarrhythmias were induced with the whole induction protocol, including triple extra stimuli and isoproterenol infusion. Based on the result of the pace mapping during the sinus rhythm, the clinical ventricular tachycardia was considered to originate at the posterolateral edge of the ostium of the left ventricular diverticulum. 99mTc-tetrofosmine and 123I-BMIPP scintigram showed low level uptake in the free wall of the diverticulum (below right), which indicated that the thin wall of the diverticulum contained active myocardium. Surgical repair of the diverticulum was recommended, but the patient chose to have an implantable cardioverter-defibrillator.