Sera from 32 consecutive patients with systemic lupus erythematosus were examined for antiheart antibodies by an immunofluorescent technique. Results were compared to those obtained with sera from five control groups of patients. Sixty-three per cent of the patients with systemic lupus erythematosus had antiheart antibodies. Antibodies were present in equal frequency in those with and those without clinical cardiac disease and were found in six of seven patients with congestive heart failure. Although both subsarcolemmal and sarcolemmal patterns of immunofluorescence were seen, the former was predominant. The clinical significance of this high prevalence of antiheart antibodies in systemic lupus erythematosus is not clear. They may represent an immunologic response to release into the circulation of altered myocardial antigens.