The predictors of long-term success during closed-chest catheter ablation of the atrioventricular junction remain unclear. Catheter ablation was performed in 32 consecutive patients, 18 male and 14 female, mean age 57 years, with intractable supraventricular tachycardia, in spite of a mean of 4.9 antiarrhythmic drugs. Duration of symptoms averaged 9.2 years, and the mean heart rate during tachycardia was 180 beats min-1. Paroxysmal atrial fibrillation or flutter was the presenting arrhythmia in 23 patients, intranodal tachycardia in four patients, and reciprocating tachycardia using an accessory pathway in five patients. There were no immediate complications, and 29 patients received a permanent transvenous pacemaker. A total of 94 shocks of 300 J (in 94% of cases) were given, (mean 2.96 shocks per patient). Chronic complete heart block was produced after one shock in nine patients (28%), and after two or more shocks (mean 3.3 +/- 1.1) in 13 patients. Modification of conduction was seen in four patients (12%). Failure to achieve any improvement of symptoms occurred in six patients (19%). There was no significant difference between the amplitude of atrial and His electrograms between patients who had complete heart block after one shock and those in whom conduction persisted. Catheter ablation was successful in 92% of patients who were given five shocks or less, but in only one of five patients (20%) who received six or more shocks. During a mean follow-up of 12 months, no patient with successful ablation during the first 24 h after catheter ablation resumed conduction or had recurrent symptomatic supraventricular tachycardia.(ABSTRACT TRUNCATED AT 250 WORDS)