The paper covers an investigation of 150 patients with infective endocarditis (IE), including 100 patients (aged 18 to 30 years old) with intravenous drug abuse as the main risk factor. This subgroup is characterized by an acute clinical course of IE, with tricuspid valve disorder in most cases and septic pulmonary embolism relapse in 72% of cases. Heart failure, multiple cardiac valvular disorder and focal lung destruction were found to be the main factors of unfavorable outcome. A relation between the size of vegetation on the heart valves and the mortality rate was established. At the same time, secondary immunodeficiency due to HIV-infection had no significant effect on the mortality rate in the group of drug addicts. More frequent cases of heart failure with systemic circulation embolism lead to higher hospital mortality in the group of patients with a subacute clinical course of IE. In elderly patients other concomitant pathology resulted in late IE detection and a high mortality rate.