Eight patients, aged 5 to 57 years, with uncontrolled symptomatic tachycardia for 2.5 to 41 years (mean 15) and significant left ventricular (LV) dysfunction in the absence of any other apparent underlying cardiac disease underwent evaluation. Incessant tachycardia was present for 0.5 to 6.0 years (mean 2.1) in 7 patients. One patient had an ectopic atrial tachycardia and 7 patients had an accessory atrioventricular pathway that participated in reciprocating tachycardia. Six patients underwent surgery; the ectopic focus was ablated in 1 patient and an accessory pathway was divided in 5 patients. One patient underwent open ablation of the His bundle and 1 patient underwent closed-chest ablation of the atrioventricular conduction system. Myocardial biopsy specimens were obtained from 5 patients, none of which yielded a specific diagnosis. Pretreatment radionuclide angiography demonstrated a mean ejection fraction (EF) of 19 +/- 9% (range 10 to 35%). Following tachycardia control a marked improvement in LV function was noted in 6 of 8 patients at rest and in 1 additional patient during exercise. The EF increased to 33 +/- 17% (range 16 to 56%) an average of 8 days after treatment and to 45 +/- 15% (range 22 to 67%) at late follow-up 3.5 +/- 40 months (mean 17) later (p less than 0.005). Seven patients remain asymptomatic 11 to 40 months (mean 22) after the corrective procedure and have resumed normal activities. These findings suggest that chronic uncontrolled tachycardia may result in significant LV dysfunction, which is reversible in some cases after control of the arrhythmia.