Paroxysmal supraventricular tachycardia (PSVT) and atrial fibrillation (AF) are both supraventricular arrhythmias, but their mechanisms are completely different. This study determines the incidence of symptomatic AF in a group of patients followed closely during routine outpatient care for PSVT. Thirty-nine patients with PSVT were followed for up to 4 years using telephone transmission of the electrocardiogram to document symptomatic arrhythmias. The cumulative proportion of patients who had AF was calculated using the Kaplan-Meier life-table method. In addition, we examined the importance of clinical variables traditionally believed to influence the occurrence of AF. The cumulative proportion of patients who had AF during follow-up was 13% at 3 months, 16% at 6 months, 22% at 1 year and 29% at 2 years. In most patients the start of AF was documented during an attack of PSVT rather than de novo as another primary arrhythmia. PSVT occurred significantly earlier during an observation period without treatment in patients in whom AF developed (p = 0.03). The occurrence of AF was not related to age, number of years of PSVT, heart rate during tachycardia or coexistent heart disease.