The paper reports on 13 cases of infectious endocarditis in the patients with prolapse of the mitral valve admitted for a period of 10 years (1979-1989) into the Clinic of Cardiology of the Fundeni Hospital. These cases stand for 3.6% of the cases with prolapse of the mitral valve admitted during that period, and 5% of the patients with infectious endocarditis. Our study dealt only with the cases of the prolapse of the mitral valve, clinically and echographically documented before the appearance of the septic graft. The hemocultures were positive in all the patients (viridans streptococci in 84.61% cases). The symptomatology, the clinical objective data and the paraclinical results (phonocardiographic, echocardiographic, electrocardiographic, radiologic, investigations with isotopes), the response to the treatment (medical, surgical) and the evolution in time were analyzed. An increase was found during endocarditis in the number of patients with holosystolic murmurs (30.7% cases) versus those with click-telesystolic murmur, the appearance in 41.15% of the cases of valvular vegetations at the Echo examination, and in 15.38% cases of ruptures of cordages. Mitral insufficiency secondary to endocarditis became worse, in 30.76% cases. The treatment with antibiotics resulted in the healing of the infection in all the cases. The surgery was not necessary in any patient during the evolution of endocarditis. The surgery (valvular prosthesis) was made in 23.07% cases, which presented, after curing the septic graft, important mitral regurgitation with cardiac insufficiency refractory to the medical treatment. Prophylaxis of the infectious endocarditis in the prolapse of mitral valve with mitral regurgitation is necessary.