The antibiotics chosen in hospitals are not always the most appropriate. A comparative prospective study with sequential analysis was designed to assess how the opinion of an infectious disease consultant affected the clinical efficacy of antimicrobial treatment in a general hospital. We studied 250 patients who had microbiologically documented infections according to Center for Disease Control criteria. The patients were divided in two groups with similar initial characteristics. Group G1 comprised 100 patients without infectious disease consultation, and group G2 contained 150 patients with infectious disease consultation. The influence of the consultant's opinion was assessed as the percentage of initially appropriate treatments, and by the number of inappropriate ones that were changed after the microbiological results were known. The appropriateness of initial treatment was significantly higher in group G2 (66% vs 55%), as were the number of treatments changed on the basis of the microbiological results (57.6% vs 33.3%). This lead to higher rates of cure, fewer deaths, and fewer cases of persistent infection in group G2. In 42.4% of G2 patients, antibiotic treatment was not changed in spite of the microbiological results and the opinion of an infectious disease consultant. We conclude that antibiotic treatment for the most clinically significant infections can be optimized if it is dispensed according to carefully developed guidelines.