OBJECTIVES: Drug costs represent a considerable cost item in the overall hospital budget. Within drug costs, oral and injectable antibiotics are indeed one of the most relevant sources of expenditure. This analysis is focused on the comparison between the actual per patient consumption of antibiotics and the Defined Daily Dose (DDD) which is frequently used as an indicator for inclusion of a drug in the Hospital Formulary and subsequent purchase. METHODS: This research was based on the analysis of the Antibiotics Request Forms (ARF's), which were forwarded by the Hospital Wards to the Pharmacy in the Geriatric Hospital “U. Sestilli”, Ancona. The analysis was focused on the daily per patient consumption (Prescribed Daily Doses, PDD) of antibiotic treatments as compared to the DDD, and on its cost (considering officially published prices and hospital discounts). RESULTS: In the period February - May, 2003, 2,350 ARF's were filed, referring to prescriptions of antibiotics. Of these, 1942 were eligible for analysis and concerned 976 patients. Total expenditure for antibiotics in such period was 63,727€; 25% of this was due to the fluorochinolones currently included in the Hospital Formulary (ATC J01MA): ciprofloxacin and levofloxacin. By comparing PDD's with DDD's, and assuming equal to one the exact correspondence between PDD and DDD of each drug, in 75% of prescriptions PDD's did not correspond to DDD's. In particular, the PDD/DDD rate resulted 0.68 for injectable ciprofloxacin, and 1.1 for injectable levofloxacin: similarly, considering oral formulations, the two rates were respectively 0.79 and 1.28. CONCLUSIONS: According to the prescription practice of this Hospital and to the type of patients seen, the inclusion in Hospital Formulary and purchase of antibiotics based on the DDD's might be misleading and could result into an incoherent management of Pharmacy stock and financial assets.