The mechanisms of cardiac involvement in systemic lupus erythematosus (SLE) were studied using immunohistochemical staining of endomyocardial biopsy specimens from 14 patients with SLE and normal coronary arteriograms. All 14 specimens showed mild interstitial edema, 11 showed mild cardiac fibrosis, and another two cases showed moderate cardiac fibrosis with myocardial derangements. Four specimens showed moderate cell infiltration in the interstitium. Area of fibrosis, diameter of myocardium and area of interstitial edema were increased in the SLE patients compared to the control cases. Immunofluorescence showed IgG and fibrinogen deposition in the membrane of cardiac myocytes and in the interstitium. Immunohistochemistry found no B lymphocytes in any of the seven SLE cases. T lymphocytes were observed in all seven SLE cases, and OKT 8 lymphocytes were increased significantly in the interstitial tissue as compared with OKT 4 lymphocytes. At endomyocardial biopsy, all 14 patients were receiving corticosteroid therapy and had low activity disease. The results suggest that cardiac tissue damage was associated with immunological abnormalities and might progress silently under conditions in which the disease activity was suppressed by corticosteroid therapy.