The clinical electrophysiologic findings in four patients with biopsy-proven polymyositis and bifascicular block are described. Atrioventricular (AV) block developed in two of these four patients, and intracardiac electrophysiologic studies documented the site of spontaneous block to be distal to the His bundle in one, whereas in the second marked His-ventricular prolongation was found, and distal block developed at low paced atrial rates. Sinus node and AV nodal function were normal in all four. These electrophysiologic data provide the first documentation of the site of AV block in patients with polymyositis and correlate well with findings of previous pathologic studies that have shown extensive fibrosis of the distal His bundle and bundle branches.