Cardiac amyliodosis is frequently associated with major electrocardiographic conduction disturbances; but that these disturbances are due to infiltrative destruction of the conduction system by amyloid is unclear. We studied the conduction systems in 23 autopsy patients with cardiac amyloidosis (group 1) (mild in seven, moderate in five and severe in 11), 21 (91 per cent) of whom had had abnormalities of conduction or rhythm during life. For comparison, we examined the conduction system in 23 control subjects matched in age and heart weight (group 2). Of the 23 patients in group 1, only three had extensive amyloidosis of the conduction system; in all three, electrocardiograms showed first degree atrioventricular block and left anterior hemiblock. A more common morphologic abnormality of the conduction system was severe sinoatrial node fibrosis present in seven (30 per cent) patients, and idiopathic atrophy and fibrosis of the bundle branches present in six (26 per cent) patients. None of the patients in group 2 had severe sinoatrial node fibrosis, but two (9 per cent) had idiopathic atrophy and fibrosis of the left bundle branch. Marked fibrosis of the sinus node was more frequent in patients with severe or moderate amyloid, but fibrosis of the bundle branch did not appear to be related to the amount of amyloid elsewhere in the heart. Varying degrees of atrioventricular and bundle branch block were also present in six patients with no morphologic abnormalities of the conduction system. Thus, conduction and rhythm disturbances are frequent in cardiac amyloidosis, but direct amyloid infiltration of the specialized conduction tissue of the heart does not account for the majority of these disturbances. Whether the increased incidence of fibrosis of the conduction system in group 1 compared to that in group 2 relates to the infiltrative myocardiopathy is uncertain.