In a prospective clinical and bacteriological study of 192 consecutive patients with septicemia admitted during the period 1967--1971 to a hospital for infectious diseases, 31 cases of proved (group I) and 10 with probably (group II) bacterial endocarditis were observed and analysed with regard to a variety of factors. The incidence of bacterial endocarditis was 3 per 1 000 admissions. The mean age of the patients was 52.6 years; there was no sex predominance. In about one third of the patients apparently normal valves were involved. Underlying non-cardiac factors were present in two thirds, even in younger age groups, in which chronic alcoholism and intravenous drug abuse were frequent. Secondary manifestations from various organs were noted in about three quarters of the patients, in one quarter already on admission. alpha-Hemolytic streptococci and staphylococci were most commonly isolated, followed by gram-negative enteric rods, beta-hemolytic streptococci, enterococci, and pneumococci. A probable portal of entry could be assumed in 80% of the patients and could often be related to the type of bacteria involved. In some cases, diagnostic or therapeutic procedures preceded the disease. A subacute course of the endocarditis was observed in one third of the patients and all these had alpha-hemolytic streptococci or enterococci. The aortic and mitral valves were equally often involved and equally often infected by alpha-hemolytic streptococci. In 4 patients, 3 of whom were drug addicts, the tricuspid valve was probably involved; all were infected by staphylococci. In one case autopsy revealed a mural endocarditis.