A prospective noninvasive electrocardiographic study was done on 50 patients with progressive systemic sclerosis. Thirty-two percent had abnormalities on the resting electrocardiogram, of which the commonest were left anterior fascicular block (16%) and first-degree heart block (8%). The 24-hour ambulatory continuous tape-recorded electrocardiograms showed serious abnormalities in 62% of the patients: supraventricular tachycardias (32%), conduction disturbances (14%), coupled ventricular extrasystoles (20%), and ventricular tachycardia (10%). Intracardiac electrophysiologic data were obtained in 20 of these patients, and functional abnormalities of the sinus node, atria, and atrioventricular node were noted in seven, nine, and 10 patients, respectively. One patient had prolonged His-Purkinje conduction time. Of the 20 patients who had electrophysiologic studies, only six had entirely normal findings. These results suggest a much higher degree of cardiac involvement in progressive systemic sclerosis than previously believed. We postulate that the first-degree heart block and supraventricular tachycardias may derive from a similar pathologic process, namely, fibrous atrophy of the proximal atrioventricular node.