A recently expressed point of view maintains that differentiation between acute and subacute bacterial endocarditis is unimportant. It is true that the histopathology of these conditions is usually indistinguishable. Contrariwise, awareness of certain differences between acute and subacute endocarditis emphasizes serious clinical and therapeutic implications. Six such differences deserve emphasis. The bacteria causing the two forms, for the most part, are different. Most cases of subacute endocarditis are caused by penicillin-sensitive Streptococcus viridans , of the mouth, which probably never produces acute endocarditis. Most cases of acute endocarditis are caused by Staphylococcus aureus , though patients infected with this organism will occasionally run a subacute course. Enterococci are versatile and may produce either acute or subacute endocarditis. The pneumococcus nearly always produces an acute infection. Second: the pre-existing heart disease. It is most unusual to find subacute bacterial endocarditis in a previously normal heart. Acute endocarditis occurs far too often in hearts