The impact of an infectious disease (ID) service on the optimal antibiotic management of 103 patients with bloodstream infections, defined as bacteremia and systemic inflammatory response syndrome, was evaluated. The optimal antibiotic management was defined according to the Sanford Guide to Antimicrobial Therapy (1996) or written internal guidelines. The judgment on optimal antibiotic management was made at the time of reporting the positive blood culture results. Switching from a broad-spectrum to a narrow-spectrum agent was carried out significantly more often by the ID service than by the attending physicians (25 of 25 vs. 20 of 40; P<0.001). In patients without empirical therapy, the ID service initiated optimal antimicrobial therapy significantly more often than physicians without training in infectious diseases (12 of 12 vs. 4 of 10, P=0.0028). Three of 12 patients in whom the attending physician misinterpreted the positive blood culture result needed 8 days to 4 months of additional hospitalization. In summary, patients for whom an ID service was provided received appropriate treatment more often and experienced significantly fewer complications.