The pattern of life-threatening bacterial infections has changed since the introduction of potent antimicrobial agents. Infections caused by pneumococci and β-hemolytic streptococci are less frequent causes of potentially fatal infections, and severe infections caused by staphylococci, other bacteria, and fungi are relatively more frequent.1-3The importance and increasing incidence of infections caused by Gram-negative bacilli have been pointed out by several investigators,1,4-7but infections caused by these microorganisms have received less attention than those due to staphylococci. This report reviews the experience with Gram-negative bacteremia during an 8-year period at the University of Illinois Research and Educational Hospitals. The etiology and some aspects of the ecology of these infections have been determined and are reported in this paper. Clinical manifestations, the effect of corticosteroid administration, and antibiotic prophylaxis and therapy of Gram-negative bacteremia are given in another report. Clinical Material All blood culture isolates of a species of