Electrograms observed prior to successful and unsuccessful ablation trials in 33 patients (362 attempts) with manifest pathways and 18 patients (194 attempts) with concealed pathways were compared to identify the electrogram patterns that are associated with successful radiofrequency ablation of accessory atrioventricular connections in young patients (mean age 12.7 years; range 4-22 years). Success was defined as permanent or transient interruption of conduction in the accessory connection. Predictors of success in patients with manifest pathways were local ventricular preexcitation (p &equals 0.0001), left-sidedness (43 or 174) of the accessory connection compared (p &equals 0.04) to right-sidedness (27 of 172), a probable Kent bundle potential (29 of 84 versus 39 of 256; p &equals 0.0001), and short antegrade atrioventricular conduction intervals (53.1 +/- 31.9 ms versus 64.6 +/- 32.0 ms; p &equals 0.02). Predictors of success in patients with concealed pathways were short ventriculoatrial conduction times (103.3 +/- 35.8 ms versus 117.9 +/- 34.8 ms; p &equals 0.01), and left-sided (42 of 125) pathways (p &equals 0.03; versus right-sided, 11 of 60). The presence of a Kent bundle potential was not significant. We conclude that specific electrogram patterns can predict successful ablation of either manifest or concealed accessory pathways. Use of these criteria may reduce the delivery of unnecessary energy to young myocardium.