Of 41 patients with Klebsiella bacteremia studied, most were newborn or elderly and only one patient had community-acquired infection. The incidence of bacteremia was evenly distributed over 16 months, except that infections in the newborns occurred in a cluster. The initial site of infection resulting in bacteremia was urinary tract (27%), gastrointestinal tract (24%), intravenous sites (20%), and pulmonary system (15%). Mortality was influenced by the underlying disease, the age of the patients, and site of initial infection. In patients with nonfatal underlying disease, deaths occurred only in patients with pulmonary or abdominal infections and did not occur in patients in whom the portal of entry was the urinary tract or intravenous sites. Single antibiotic therapy with aminoglycoside was adequate in this latter group.