Accelerated idioventricular rhythm (AIVR) has been reported in patients with acute myocardial infarction, digitalis excess, and subarachnoid hemorrhage, and in patients with rheumatic, primary myocardial, and hypertensive heart disease. Discovery of AIVR in 2 patients without heart disease led us to review reports from 700 Holter monitor studies. Seven patients without recent myocardial infarction were studied retrospectively. Three of the 7 had no evidence of heart disease; 5 of the 7 had abnormalities of the central nervous system. Examples of AIVR show approximation of the sinus rate and ectopic rate; onset and offset occur abruptly or with sinus rate slowing and fusion beats. One patient remained in AIVR for up to 10 minutes accompanied by retrograde atrial capture. The rhythm's acceleration with exercise suggests that it is under autonomic influence, a phenomenon also seen in CNS stimulation studies in dogs. AIVR occurs infrequently in patients without demonstrable heart disease. Our experience suggests a good prognosis, but further study is needed onthe natural history of AIVR in asymptomatic patients and on the necessity of treatment.