Potent sodium channel, blockade with type IC antiarrhythmics can provoke characteristic electrocardiographic changes consistent with Brugada's syndrome in unselected patients with atrial fibrillation. In 176 treated patients, the incidence of the characteristic abnormality is small (2.3%), and thus although no ventricular arrhythmia events were observed during follow-up, the long-term clinical significance in a larger patient cohort remains to be determined. These data provide insight into the specificity of the response to type I drug administration in patients suspected of having Brugada's syndrome.