The Kearns-Sayre syndrome is combined with a progressive external ophthalmoplegia (PEO), retinal pigmentary degeneration and heart block. In some patients, progression of incomplete forms has been described and potentially fatal conduction disturbances may occur. The disease is considered as a systemic mitochondrial disorder. As part of an ongoing prospective study 6 patients (3 female, 3 male; age 32 +/- 9 years) with PEO and typical ultrastructural changes of a mitochondrial myopathy in their skeletal muscle were examined. The ECG disclosed atrio/intraventricular conduction defects in 5 patients: 1 patient had a third degree AV block which was treated by a pacemaker. Another patient had left anterior fascicular block with complete right bundle branch block. In 3 other patients an incomplete right bundle branch block was registered. In 1 patient, His-bundle electrography disclosed a block distal to His by atrial high rate pacing before and after i.v. injection of ajmaline; prophylactically a pacemaker was implanted. The mean HV-interval increased significantly under ajmaline by 44% in patients with sinus rhythm. Hemodynamic studies were normal in 5 patients at rest; only 1 patient was abnormal during exercise. Myocardial biopsy demonstrated mitochondrial abnormalities such as variability in shape and size. In 4 patients concentric cristae mitochondriales were seen. Our results suggest that atrioventricular conduction defects are common in patients with PEO. By means of endomyocardial catheterbiopsy a mitochondrial cardiomyopathy could be detected.