At first, the tachycardia was regular with a narrow QRS and a retrograde P-wave at the end of the QRS complex. Then, after spontaneous interruption, there was a preexcited first sinus beat, a pattern characteristic of left-sided accessory pathway. This was followed by normal sinus rhythm, without preexcitation. The diagnosis was orthodromic tachycardia mediated by a left-sided accessory pathway with bidirectional conduction and intermittent preexcitation. Ablation of the accessory pathway was performed.