Risk stratification in patients with Wolff-Parkinson-White (WPW) syndrome is an important clinical problem given a small, but significant risk of sudden death. It has been widely accepted that intermittent pre-excitation is associated with a low-risk pathway. We report a case of WPW with intermittent pre-excitation at baseline, but with marked sensitivity to adrenergic stimulation, revealed by exercise and isoproterenol administration.