Ambulatory electrocardiography was performed in 183 patients with systemic sclerosis recruited from five centers who were selected to reflect a balanced population with respect to disease extent and duration. Ventricular ectopy occurred in 67 percent of patients and was strongly correlated by both univariate and multivariate analyses with total mortality and with sudden death. By multivariate analysis, ventricular ectopy was strongly associated with increasing patient age and with other evidence of cardiac and pulmonary involvement but not with clinical and laboratory measures of duration and extent of systemic sclerosis. Evidence of myocardial fibrosis thought to be secondary to microvascular alteration is common in systemic sclerosis, but the clinical implications of myocardial involvement are less well appreciated. The present data suggest the need for ambulatory electrocardiography in the clinical assessment of selected patients with systemic sclerosis, especially those with cardiac or pulmonary involvement, as well as for studies of the effects of antiarrhythmic therapy.