Intraoperative epicardial mapping data obtained in 73 consecutivepatients operated upon for the Wolff-Parkinson-White syndrome werereviewed. Fifty-six patients had single and 17 patients had multipleaccessory pathways. Except for right free wall pathways, all bypass tractswere divided using an endocardial approach. There were 2 operative deaths,1 of which occurred after a concomitant mitral valve replacement. A totalof 78 of the 87 pathways present in the 71 survivors were successfullyablated (90%). All failures occurred in patients with left posterior septalpathways. Epicardial mapping performed prior to bypass was found helpful inidentifying multiple distinct accessory pathways which had been missedpreoperatively. This occurred in 6 patients and led to appropriatecombinations of classic operative approaches which resulted in all of thesepathways being successfully divided. Further, by demonstrating that Kentbundles often presented as multiple closely-spaced or arborized accessorypathways, intraoperative mapping led to widening of the margins of surgicaldissection, and in particular to an additional left atriotomy in all casesof left posterior septal accessory pathways which resulted in a substantialimprovement in our rate of success.