Molecular mimicry between pathogen and host has been proposed as a mechanism for the development of autoimmune diseases. Evidence suggests that microorganisms contain proteins which are similar enough to host proteins that they can stimulate existing B and T cells to respond to self proteins. The loss of immune regulation during responses against microbial antigens may explain development of pathogenic B and T cell responses in autoimmune diseases associated with infections. The study of B and T cell responses against the group A streptococcal antigens, N-acetyl-glucosamine, M protein and the autoantigen cardiac myosin has led to a better understanding of how molecular mimicry may play a role in disease. Studies of human monoclonal antibodies, T cell responses and animal models in comparison with the immunopathology in the human disease has provided information about the steps leading to inflammatory heart disease in autoimmune post-streptococcal rheumatic carditis. The new data indicate that the steps in pathogenesis of rheumatic heart disease following group A streptococcal infection include the following events. First, the development of crossreactive autoantibodies against the group A streptococcal carbohydrate antigen N-acetyl-glucosamine and cardiac myosin. Second, these antibodies react with valvular endothelium which becomes inflamed with expression of vascular cell adhesion molecule-1 (VCAM-1). After this event, T cells, CD4+ and CD8+, infiltrate through the endothelium/endocardium into the valve which is an avascular structure. Aschoff bodies or granulomatous lesions may form containing macrophages and T cells underneath the endocardium. The T cells are responsive to streptococcal M protein antigen sequences. The valve becomes scarred with eventual neovascularization and progressive, chronic disease in the valve. In the host, the mimicking antigens cardiac myosin and laminin have been involved in the myocardium and valve, respectively. As in other autoimmune diseases, both environmental and genetic factors are involved in the development of rheumatic carditis and inflammatory heart disease, a result of mimicry between the group A streptococcus and heart.