We analyzed the clinical characteristics of 10 patients with pneumococcal endocarditis hospitalized between 1974 and 1984. Patients with pneumococcal endocarditis were typically middle-aged men. Forty percent were alcoholic. They sought medical attention early in the course of their illness and were given appropriate antibiotics promptly. The aortic valve was involved in seven patients. Five patients developed signs of severe valvular insufficiency, and congestive heart failure was present at the time of admission in four patients. Only three patients were recognized to have endocarditis prior to death or to the occurrence of a major complication of their infection. The total in-hospital mortality rate among these patients was 50%. Thus pneumococcal endocarditis is generally an acute, left-sided endocarditis that is associated with rapid valvular destruction and a high mortality rate. Unfortunately, recent advances in diagnosis and treatment of bacterial endocarditis have not substantially improved the outcome of this devastating infection.