To examine the relationships between bacteremia and severe sepsis and assess the influence of characteristics of infection on the risk of severe sepsis and outcome of bacteremia, we analyzed all clinically significant episodes of bacteremia occurring during a 2-mo prospective survey of 85,750 admissions to adult wards and intensive care units (ICUs) of 24 hospitals in France. Incidence rates of bacteremia and of bacteremic severe sepsis were 9.8 (95% CI: 9.2 to 10.5) and 2.6 (95% CI: 2.2 to 2.9), respectively, per 1,000 adult admissions; these rates were eight and 32 times higher in ICUs than in wards, respectively. Independent risk factors for severe sepsis during bacteremia included age (p < 0.001) and an intraabdominal (p < 0.001), pulmonary (p < 0.001), neuromeningeal (p = 0.004), or multiple (p < 0.001) source of bacteremia, but not categories of organisms involved. The probability of death at 28 d after bacteremia was 25% (95% CI: 23% to 29%), and was 54% (95% CI: 48% to 61%) in patients with bacteremic severe sepsis. The risk of death after bacteremia increased with age (p < 0.001), a rapidly or ultimately fatal underlying disease (p < 0.001), and the presence of severe sepsis (p < 0.001), shock (p = 0.03), and infection caused by gram-positive organisms other than coagulase-negative staphylococci, relative to other organisms (p < 0.001). A primary urinary tract source of infection was associated with a better prognosis (p = 0.03). We conclude that whereas sources of infection influence both the risk of severe sepsis and the outcome of bacteremia, the microbiologic characteristics of infection influence only the outcome, with gram-negative organisms and coagulase-negative staphylococci posing a lesser risk than other organisms.