One hundred and five cases of bacteremia due to Staphylococcus aureus were reviewed to assess the current clinical spectrum of serious staphylococcal disease. Mortality was 21 percent, lower than previously reported. Patients could be separated into two groups according to the presence of identifiable primary staphylococcal infections; 63 bacteremic patients had such lesions, the remaining 42 lacked them. The latter group contained 24 of 26 cases of endocarditis. Illnesses in that group were marked by the presence (in 38 of 42 patients) of staphylococcal foci occurring secondary to bacteremia. Such foci were responsible for five of seven instances of relapse or treatment failure encountered in that group. Secondary staphylococcal foci occurred in only five of 63 patients with primary infections, and the response of this group to conventional therapy for bacteremia was satisfactory. This study suggests that endocarditis has become an unusual complication of identifiable primary staphylococcal infection. A clinical classification based on the presence of such lesions therefore separates bacteremic patients likely to be cured by conventional antibiotic therapy (those with primary infections but no secondary foci) from others (those with secondary foci, suggesting endocarditis) who should receive a more prolonged course of antibiotics.