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Adenosine has been shown to reliably confirm the success of accessory pathway cathater ablation by producing transient atrioventricular (AV) block during atrial and ventricular pacing. This is due to the insensitivity of accessory of pathway conduction to adenosine with the rare exception of accessory pathways with decremental conduction properties). However, 4 of 204 consecutive patients who underwent successful accessory pathway ablation (as shown by adenosine-induced transient AV block) had recurrent AV reciprocating tachycardia involving a second, previously nonmanifest accessory pathway. In each case, the second accessory pathway was localized to a site disparate from the original pathway. No pathway showed decremental anterograde or retrograde conduction properties. In 2 patients, adenosine initially did not show the presence of the second concealed accessory pathway, because the refractory period of the accessory pathway was longer the pacing cycle length used to assess ventriculoatrial conduction. Only when the refractory period of this second accessory pathway was shortened by infusion of isoproterenol did adenosine reveal the presence of the pathway during follow-up electrophysiologic study. In another patient, a nondecremental accessory pathway was shown to be sensitive to adenosine. In the remaining patient, the second accessory pathway may have been transiently injured during the initial study, thereby simulating adenosine sensitivity. Therefore, it is concluded that (1) adenosine is a highly, but not completely, effective method for immediately assessing the efficacy of accessory pathway catheter ablation; (2) concomitant infusion of isoproterenol during adenosine administration is necessary to recognize the presence of accessory pathways with prolonged refractory periods; and (3) assessment of the presence of pathway conduction with adenosine after ablation should preferably be performed at the longest possible, paced cycle length.