The main results of the bloodstream infection (BSI) component of the Belgian National Programme for the Surveillance of Hospital Infections (NSIH project) are reported. From October 1992 to September 1996, 117 hospitals (59.1% of Belgian acute-care institutions) reported 13678 nosocomial BSIs. The incidence was 7.05 BSI episodes per 10000 patient-days. The incidence of BSI increased with hospital size and over time. Bloodstream infections were secondary to an infectious body site in 40.3% of the episodes, catheter-related in 23.5%, and of unknown origin in 36.2%. The associated in-hospital mortality was 31.4% and was highest in BSIs secondary to a respiratory tract infection (49.3%). In intensive care units, the incidence of BSI was 38.5 per 10000 patient-days. Coagulase-negative staphylococci were the most prevalent microorganisms (22%), followed by Staphylococcus aureus (14.1%) and Escherichia coli (13.5%). In catheter-related BSIs, these proportions were 41.9%, 18.8%, and 2.3%, respectively. The proportion of polymicrobial episodes was 9.9%. Methicillin resistance in Staphylococcus aureus was 22.3%. With its high participation rate, the NSIH project has characterized the epidemiology of nosocomial BSIs in Belgium during the period studied.