Swan-Ganz catheterization was performed because of persistent unexplained tachycardia in a patient with myocardial infarction. Although there was no premonitory arrhythmia, ventricular tachycardia developed as the catheter was removed. Direct-current cardioversion was required. The occurrence of ventricular tachycardia requiring cardioversion in a patient who experienced no ventricular ectopy earlier in his hospitalization shows that Swan-Ganz catheter removal may provoke potentially serious arrhythmia. Uninterrupted vigilance from the time the Swan-Ganz catheter is inserted to the time it is removed is essential. Therefore, we recommend that the catheter be removed under ECG monitoring by experienced personnel, with lidocaine and a defibrillator available.