The group B streptococcus has been shown to be a major cause of meningitis in the newborn and an occasional cause of endocarditis and sepsis in postpartum women. Little attention has been devoted to this organism as a cause of bacterial endocarditis. Twelve patients with group B streptococcal endocarditis were seen at The Presbyterian Hospital, New York, NY, between 1974 and 1985. There were seven women, five men. Ages ranged from 32 to 81 years. Serious underlying disease was present in all - diabetes mellitus in seven, carcinoma in three (bladder in two, and breast in one), alcoholism in three, malnutrition in two, heroin addiction in one, tuberculosis in one, serious prior valvular heart disease in two. The aortic valve was affected in four patients - mitral in two, mitral and aortic in one, tricuspid in four, unknown in one. The presentation was acute in seven patients. Metastatic infection occurred in seven, heart failure in six, major emboli in four, septic pericarditis in one, myocardial abscess in one. The group B streptococcus should be considered as a pathogen capable of causing acute endocarditis in certain patients with defects of host defense, particularly patients with diabetes mellitus, carcinoma or alcoholism. Cardiac surgery may be necessary in these patients due to the rapid destruction of the valves which occurs, in spite of the fact that the organisms are usually highly susceptible to penicillin.