Transthoracic external cardioversion is, if carried out properly, a highly effective cardioversion modality for most patients with atrial fibrillation. Failure to return to sinus rhythm (even for one or two beats) can be related to high transthoracic impedance. Internal cardioversion may offer a safe and worthwhile alternative for these patients, in need of sinus rhythm. This technique, mostly requiring only mild sedation, is actually performed in many electrophysiology centers. Because recurrence rates of atrial fibrillation seem to be the same for both methods, this invasive technique should be considered in patients with a rather low chance of atrial fibrillation recurrences. We present a case report of an extremely obese woman, resistant to external cardioversion. She successfully underwent internal low-energy cardioversion.