Immunopathologic studies were performed on cardiac tissue obtained at autopsy in 10 patients with severe systemic lupus erythematosus (SLE). The immunopathologic findings were correlated with histopathologic and clinical evidence of cardiac injury, and with clinical and serologic features of SLE. Immune reactants were demonstrated by direct immunofluorescence in nine patients in a granular deposition pattern suggesting immune complex aggregates. Histologic and gross anatomic findings of inflammation were generally more focal than was the distribution of immune reactants. Most of the immune deposits were present in the walls of the blood vessels of myocardium (eight of 10) or pericardium (two of three). In one patient with Libman-Sacks endocarditis, immunoglobulin and complement components were present in the valve stroma and the vegetations. The immune deposits around epicardial nerve fibers in two patients with severe neurologic manifestations contained immunoglobulin E(IgE). In general, the most intense and widespread immune deposits were observed in patients with persistently increased serologic and clinical evidence of activity of their systemic disease. These results suggest a role for immune complex deposition in the pathogenesis of the cardiac lesions of SLE.