The cases of all patients hospitalized at Boston City Hospital during 1972 who had blood cultures positive for a clinically significant, aerobic bacterial pathogen or for Candida were analyzed with respect to incidence and mortality, sex, age, admission to medical or surgical services, and the causative organism. Similar data were obtained for 11 years between 1935 and 1969 selected to reflect the introduction and general use of various effective antibacterial agents. Comparisons were also made between hospital-acquired bacteremic infections (defined as those in which the first positive blood culture was obtained on or after the third day in the hospital) and community-acquired infections (defined as those with positive blood cultures on admission or within the first two days in the hospital). In 1972, the incidence of bacteremic infections (but not the case-fatality ratio) was significantly higher in males than in females. Bacteremic infections were more than twice as frequent on the medical than on the surgical services, but the case-fatality ratio was slightly but not significantly higher on the surgical services. Bacteremia wasteremia was most frequent in the youngest (birth through nine years) and the oldest (greater than or equal to 60 years) age groups, whereas the case-fatality ratio was lowest in the youngest group and increased with each decade of life. Streptococcus pneumoniae was the most frequent organism causing bacteremia; next were Escherichia coli, Klebsiella-Enterobacter, and Staphylococcus aureus, in that order...