The impact on antibiotic therapy of the first notification of positive blood cultures was assessed for 735 episodes of bacteraemia detected during 1992 in the County of Northern Jutland, Denmark. A primary focus of infection was defined in 498 episodes, the urinary tract being the most frequent (n = 182, 25%). Twenty-nine patients (3.5%) had died prior to the initial contact. In 12 episodes antibiotic therapy had either been stopped or data were not available, leaving 694 episodes for further assessment. In 567 episodes antibiotic therapy had been started prior to the initial contact, the most frequent regimen being ampicillin or an ampicillin-aminoglycoside combination (295 episodes), whereas cephalosporins, thienamycin, and fluoroquinolones were seldom used (41 episodes). The ongoing antibiotic coverage was deemed appropriate in 418 episodes (60%), non-optimal in 90 (13%), and lacking in 186 (27%). The notification of positive blood cultures elicited changes in antibiotic therapy in 315 episodes (45%), including commencement of antibiotic therapy in 127 (18%). Thus, blood culture results have a measurable impact on antibiotic therapy.