A large increase in the use of wide-spectrum antibiotics has occurred in Spain. The objective of our study was to evaluate the influence of applying protocols for the use of such antibiotics. A prospective, observational study was carried out based on the information provided by the pharmacy service about prescriptions of ceftriaxone, cefotaxime, ceftazidime, imipenem and ciprofloxacin at the single-dosing department of the hospital. Two groups were included in the study: 1) control group (October-December 1995), which did not take into account the protocols for antibiotic treatment; and 2) the study group, during the same months in 1996, which took into account the consensual protocols. The criteria for the type of patient, infection and outcome, and type of antibiotic treatment were evaluated according to the rules of the international literature for this type of study. The baseline characteristics of both groups of patients were similar. Following the application of the protocol, a decrease in the use of antibiotics was observed (108 cases in group 2 vs. 207 cases in group 1); an increase in the appropriateness of treatment (50.72% in group vs. 77.57% in group 2); an increase in the cure rate (78.26% in group 1 vs. 91.67% in group 2); and a decrease of 5.01 days in the length of hospital stay (22.99 days in group 1 vs. 17.98 days in group 2), which was statistically significant, and thus led to a savings of 2073.74 Euros per cured patient. The application and adherence to a consensual protocol is a beneficial method of using antibiotics since it improves the quality of their use and makes it more cost-effective.