Intravenous verapamil was given to two haemodynamically stable patients with persistent atrial tachycardia, resulting in circulatory arrest requiring CPR in one and collapse with unrecordable blood pressure in the other. Both responded to resuscitation and tachycardia was subsequently controlled with propranolol in one and sotalol in the other. Factors contributing to the cardiovascular collapse included: (i) left ventricular dysfunction; and (ii) failure to convert the tachycardia to sinus rhythm. It was concluded that verapamil may be dangerous in supraventricular tachycardia not due to atrioventricular (AV) junctional re-entry, despite normal blood pressure and perfusion, particularly if left ventricular dysfunction were present. If the diagnosis of AV junctional re-entry is in doubt, adenosine is preferable as it is less likely to cause haemodynamic collapse and will assist in making the diagnosis.