The DDD/100 patient days data format overestimated antibiotic use density changes in this hospital both in medicine (81% vs 48%) as well as in surgery (69% vs 39%) when compared with PDD/100 patient days. Due to changes in the number of admissions and length of stay over the years, the percent change between the two periods expressed in doses per 100 patient days in addition differed substantially from that estimated by using the DDD per admission or PDD per admission data format. Studies evaluating the evolution of hospital antibiotic use need to address the limitations and adequacy of the different data formats.