Five hundred episodes of septicemia were reviewed, with emphasis on laboratory and epidemiologic findings. The isolation of facultative and anaerobic gram-negative bacilli, fungi, and gram-positive cocci (except viridans streptococci and Staphylococcus epidermidis) almost always indicated true bacteremia, whereas the isolation of aerobic and anaerobic gram-positive bacilli, including Clostridium species, often represented contamination. More than 99% of all episodes were detected when two samples of blood (a total of 30 ml) were cultured. The five most common isolates were Escherichia coli, Staphylococcus aureus, Streptococcus pneumoniae, Klebsiella pneumoniae, and Pseudomonas aeruginosa. The incidence of septicemia was highest among medical patients and lowest among obstetric-gynecologic patients. Two-thirds of all episodes were nosocomial; S. aureus, enterococci, facultative gram-negative bacilli, and fungi were especially common nosocomial pathogens. The microorganisms isolated varied with the hospital service; polymicrobial episodes were especially common among surgical patients and transplant recipients. The most common sources of bacteremia were the respiratory, genitourinary, and gastrointestinal tracts; however, the source was unknown in nearly one-third of episodes. Microorganisms causing septicemia in neutropenic and nonneutropenic patients were not different; however, polymicrobial infections were more frequent in the presence of neutropenia. After antimicrobial susceptibility data became available, therapy was appropriate greater than 90% of the time.