We describe a simple method to unmask unapparent pre-excitations at the emergency room. This method allows a non-invasive and definite diagnosis of accessory pathways capable of anterograde conduction. In patients presenting with paroxysmal SVT, close inspection of the electrocardiogram during sinus rhythm should look for a pre-excitation. Unfortunately, a pre-excitation can be completely unapparent during sinus rhythm despite the presence of an antegradely conducting accessory pathway. This is typically the case in the so-called Mahaim fibers. These bypass tracts give rise to antidromic atrioventricular reentrant tachycardias, in which the bypass tract serves as the anterograde limb of the circuit and the atrioventricular node as the retrograde limb of the reentrant circuit. In these patients, it is easy to make a misdiagnosis of supraventricular tachycardia with functional left bundle branch block. Indeed, due to the slow and decremental conduction properties of these bypass tracts, the baseline ECG is either normal or shows only subtle pre-excitation. Herein, we describe a simple tip, consisting of two successive administrations of adenosine (during ongoing tachycardia [upper panel] and during sinus rhythm [lower panel]), to make a non-invasive and definite diagnosis of this condition at the emergency department. By blocking the anterograde conduction over the atrioventricular node, adenosine infusion during sinus rhythm will indeed result in ventricular activation occurring exclusively over the accessory pathway. Importantly, the use of this simple method to unmask accessory pathways capable of anterograde conduction is not limited to the diagnosis of Mahaim fibers. Indeed, it can be used more generally in patients presenting with paroxysmal SVT whenever the ECG following tachycardia termination does not definitely rule out the presence of an accessory pathway (Figure 1).