Early antimicrobial treatment has a great influence on the outcome of patients with blood stream infections (BSI). The study was designed to see if the simple practice of patient categorization (community acquired, nosocomial or infection in haematological unit) combined with Gram stain data could be used to guide empirical treatment of BSI in 1901 consecutive positive blood culture findings. There were considerable differences in the occurrence of common pathogens and their antimicrobial susceptibilities between patient categories especially for Gram-positive cocci. For example, second generation cephalosporins covered more than 70% cocci in clusters and over 80% of cocci in chains in community acquired infections whereas in hospital acquired infections the corresponding figures were only 47 and 44%. We conclude that Gram stain results of positive blood cultures along with the knowledge of where the infection was acquired, would allow early accurate targeting of antimicrobial therapy for BSI.