Paroxysmal supraventricular tachycardia (PSVT) is a common clinical problem. Valsalva maneuver and adenosine are effective therapies for many patients with PSVT, although any conversion to an irregular or wide complex tachycardia should prompt consideration of a preexcitation syndrome. We report a case where the Valsalva maneuver and adenosine, in a patient with PSVT and previously undiagnosed Wolff-Parkinson-White syndrome, caused atrial fibrillation and led to a haemodynamically unstable wide complex tachycardia and ventricular fibrillation. In PSVT, where preexcitation has not been excluded, the Valsalva maneuver and adenosine can be potentially dangerous.