In order to assess the occurrence and clinical significance of abnormal electrograms in idiopathic dilated cardiomyopathy, endocardial electrode mapping during sinus rhythm and programmed ventricular stimulation were performed in 52 patients with or without clinical ventricular tachycardia. Abnormal endocardial electrograms were recorded in 77% of the patients and were diffusely distributed over the entire left ventricular endocardium. No relation could be established between the occurrence of late potentials or fractionations and clinical or induced arrhythmias. Endomyocardial biopsy samples were taken from 20 patients and showed that reduced myofibril volume fraction was related to the occurrence of abnormal endocardial electrograms. Neither induced arrhythmias nor the presence of late potentials or fractionations identified patients who died of sudden cardiac death during the mean (SD) follow up of 33 (11) months. Thus abnormal endocardial electrograms recorded during sinus rhythm in idiopathic dilated cardiomyopathy may only be interpreted as being a sign of damage to the myocardial cells.