The aim of the study was the comparison of clinical, laboratory end instrumental data between systemic lupus erythematosus (SLE) end infectious endocarditis (IE) for analysis of similar and different features. Clinical, laboratory and instrumental data were analysed and compared for 96 SLE and 23 IE patients. SLE and IE have the following common features: fever, pleurisy, pericarditis, myocarditis hemorrhagic vasculitis, renal disorders, anaemia, concentrations of circulating immune complex (CIC), IgM. Characteristic of SLE were skin erythema, alopecia, cerebrovasculitis, lymhadenopathy, pneumonitis, frequent articular lesions, pancytopenia, high IgG levels, and antibodies to DNA. After echocardiographic investigations a rare revelation of the damage of endocardium, and the lack of destruction of the valve were detected. IE was characterized by thromboembolic complications, splenomegaly, pneumonia, high IgM levels, high incidence rate of RF, positive hemoculture. By echocardiographic studies bacterial vegetations end valvular pathology were detected. These data gives us the possibility for early differential diagnostics of these two diseases.