Factors predictive of a fatal outcome were retrospectively studied in 248 patients admitted with Staphylococcus aureus septicemia during 1965-1982, 78 of whom had endocarditis. 77 patients were intravenous drug addicts and 47 of them had endocarditis. 48 patients (19.4%) died. The fatality rate in addicts and non-addicts from septicemia was 0% and 17.9% and from endocarditis 8.5% and 61.3%, respectively. After analyzing clinical and laboratory data available early in the course of the disease 4 risk factors were found both in septicemia and endocarditis: age greater than or equal to 60 yr, pre-existing cardiovascular disease, prior hospitalization within 30 days of onset of illness, and neurological symptoms and/or signs. In addition, in endocarditis a platelet count before therapy less than 100 X 10(9)/l and left-sided involvement were unfavourable prognostic factors.