Disorders of rhythm and conduction are characteristic of the cardiac involvement in progressive systemic sclerosis (PSS), but their over-all frequency in PSS is not well established. Therefore, 46 ambulatory patients with PSS underwent several tests of cardiopulmonary function, including a 24-hour continuous electrocardiogram (Holter monitor). Conduction disturbances (sinus node dysfunction, first-degree heart block, pre-excitation), supraventricular arrhythmias (supraventricular tachycardia, atrial fibrillation, premature contractions of atrial or junctional origin) and ventricular arrhythmias (ventricular tachycardia, multifocal premature contractions) were observed on Holter monitoring in 26 subjects. Although these arrhythmias and conduction disorders were predictably observed in patients who complained of palpitations or syncope, or who had an electrocardiogram which showed first-degree heart block, ventricular bigeminy, left anterior superior hemiblock, prolonged p wave, right or left axis deviation, right or left ventricular hypertrophy, pathologic Q waves or low voltage, they were often found in patients who lacked other clinical evidences of heart disease. Arrhythmias and conduction disturbances were not significantly more frequent among patients with cardiomegaly or interstitial change on chest roentgenogram nor were they related to the presence or severity of abnormal lung function. This study suggests that Holter monitoring may be a valuable adjunct in evaluating heart disease in PSS.