Atrial fibrillation was identified as the initial arrhythmia complicating the Wolff-Parkinson-White syndrome in ten (9%) of 108 patients. Despite initially rapid ventricular responses in seven, long term survival and control of arrhythmia were excellent on medical treatment. Whereas symptom free patients with the pre-excitation syndrome who have additional underlying disease predisposing to atrial fibrillation may need detailed electrophysiological study, a more conservative approach is suitable for the typical symptom free individual.