Therapeutic hypothermia has been used for neuroprotection following cardiac arrest presenting with ventricular tachycardia or ventricular fibrillation regardless of underlying cause. Long QT syndrome is a cause for polymorphic ventricular tachycardia, and we know that therapeutic hypothermia increases the QT interval. We managed a 27-year-old woman, who was 10 weeks post-partum, who collapsed secondary to ventricular fibrillation at home. Bystander cardiopulmonary resuscitation was started with successful resuscitation after a rescue shock from paramedics. On hospital admission, her computerised tomography head, computerised tomography pulmonary angiogram and echocardiography did not show any abnormality. Her baseline electrocardiogram showed prolonged QTc interval of 504 ms without ischaemic changes. After intubation and ventilation, she was treated with therapeutic hypothermia for 48 h. She had a further episode of polymorphic ventricular tachycardia requiring rescue shock just prior to starting therapeutic hypothermia in hospital. No dysrhythmias occurred during therapeutic hypothermia, although the QTc further increased. After stopping the therapeutic hypothermia, she had two further ventricular tachycardia episodes. After commencement of beta blockers, she remained free of arrhythmias, and an implantable cardioverter defibrillator was implanted, she has recovered without any neurological deficit. Ventricular dysrhythmias caused by prolongation of the QT interval during or after therapeutic hypothermia are not well understood. There has been a report of a patient also having ventricular dysrhythmia 2 h after re-warming post therapeutic hypothermia and also a report of arrhythmia free period during therapeutic hypothermia in a long QT syndrome patient; both these features are present in our patient. Re-warming is not usually known to cause any arrhythmias; however, it could be a problem in those with long QT syndrome. Whether therapeutic hypothermia has a place in helping to control ventricular dysrhythmias needs further study.