The inappropriate use of antibiotics has been documented by several studies. Most of these studies have been performed in the USA and there is very little data from Europe and Switzerland. A three-month prospective study was carried out at the University Medical Center of Lausanne in the departments of internal medicine, general surgery and traumatology. During this period, 2112 patients were hospitalized among whom 878 received one or more courses of antibiotic therapy: the treatment was given prophylactically in 35% of the cases and therapeutically in the other 65%. A total of 1165 antibiotic prescriptions were evaluated. The administration of the antibiotic was considered to be indicated in 96% of the cases. However, 24% of the prescriptions contained at least one error. 50% of the errors were observed in prophylactic treatments and most were related to the length of administration and the spectrum of the agents. The errors in therapeutic prescription were found mainly in indications for urinary and respiratory tract infections, and were related to the spectrum of the antibiotic and the length of treatment. It was calculated that the inappropriate use of antibiotics accounted for approximately 7% of the total costs of all antibiotics used. We estimated that half of this excess in costs could probably be saved by the introduction of simple corrective measures mainly directed at limiting the duration of administration.