Introduction - To prepare for a parallel surveillance of hospital antibiotic use and trends in bacterial resistance, we analyzed antibiotic use at the Assistance-Publique - Hôpitaux de Paris (AP-HP) over a 15-year period. Methods - Annual orders billed to the 38 AP-HP hospitals from 1990 to 2004 were collected and expressed in WHO-defined daily doses (DDD) for each antibiotic. These were reported relative to the number of hospital days (HD) or admissions. Results - Total DDJ consumption decreased by 10.7% over 15 years. In terms of DDJ per 1,000 HD, it increased by 11.3% before stabilizing after 1997 at around 450 DDJ/1,000 HD; when expressed per 1,000 admissions lasting more than 24 hours, however, it decreased by 10.4%. Significant variations were observed between and within the different classes of antibiotics: an increase in amoxicillin-clavulanic acid (+30%), third-generation cephalosporins (+33%), and fluoroquinolones (+37%), and a decrease in macrolides (-50%) and aminoglycosides (-41%). Conclusions - While it is necessary to monitor changes in antibiotic consumption over extended periods, these changes result from two factors that make interpretation difficult: 1) significant variations in medical activity (increase in activity lasting less than 24 hours, reduction in length of stay); 2) changes in practice. Choosing the denominator for the consumption indicator is therefore difficult. (R.A.)