In this report we present an unusual case of a 45-year-old female patient with systemic lupus erythematosus (SLE) who was hospitalized for mitral valve replacement. In her childhood she presented with mitral stenosis and chorea on which grounds a preliminary diagnosis of rheumatic fever was established. After a quiescent period lasting two decades her disease erupted with mitral stenosis, thromboembolic phenomena, and nephritis. Due to severe malfunctioning of her mitral valve, the patient eventually underwent mitral valve replacement. The antibodies involved in the pathogenesis of our patient's valvular disease were studied by immunohistochemical analysis, applying rabbit polyclonal anti-human IgG and IgM anti-human C3c and anti-idiotypes to a mouse monoclonal naturally occurring polyspecific human monoclonal anti-cardiolipin antibody termed S2.9, and to the 16/6 Id which defines a common Id on anti-DNA antibodies in patients with SLE. Immunoperoxidase staining using an anti-idiotype mAb to anti-cardiolipin antibodies demonstrated the deposition of these anti-bodies in the subendothelial layer of the valve. We believe that anti-phospholipid syndrome (APS) with SLE was the initial and primary disease in this patient. These findings clearly indicate that APS must be considered in the differential diagnosis of rheumatic fever, particularly in young female patients who present with mitral stenosis and chorea.