Biventricular hypertrophy and failure developed in two patients during treatment of systemic lupus erythematosus with chloroquine phosphate. In both patients, morphologic analysis of the myocardium, obtained by a right ventricular endomyocardial biopsy in one patient and at autopsy in the other, revealed accumulations of electron-dense concentric and parallel lamellae and curvilinear bodies within cardiac myocytes. These deposits were similar to those reported in chloroquine-induced skeletal myopathy and were considered to represent evidence of chloroquine-induced cardiotoxicity rather than a cardiovascular manifestation of the underlying disease. Clinical awareness and an endomyocardial biopsy specimen are necessary for the appropriate diagnosis of chloroquine-induced cardiomyopathy.