It is generally assumed that the antibiotic prescription policy of a hospital has a significant impact on bacterial resistance rates; however, few studies are available to support this concept with valid statistical data. During a 3-year period from 1997 to 2000, we monitored the consumption of ␤-lactam and other antibiotics with known activity against Pseudomonas aeruginosa in a 600-bed community hospital. Monthly isolations of P. aeruginosa were assessed, and resistance rates were recorded. Partial correlation coefficients between consumption and resistance rates were determined, taking into account possible associations with other variables such as seasonal effects and transfers from other hospitals. A total of 30 ؎ 7 novel P. aeruginosa strains per month were isolated without epidemic clustering. Prescriptions of imipenem varied significantly during the study period, while prescriptions of other antipseudomonal agents were stable, with the exception of an increase in piperacillin-tazobactam prescriptions. Rates of resistance of P. aeruginosa to the antimicrobial agents used showed a time course similar to figures for imipenem consumption. Monthly rates of resistance to imipenem (partial correlation coefficient [cc], 0.63), piperacillin-tazobactam (cc, 0.57), and ceftazidime (cc, 0.56) were significantly associated with imipenem prescription rates in the same or the preceding month, while consumption of ceftazidime or piperacillin-tazobactam had no apparent association with resistance. Among the variables investigated, imipenem consumption was identified as the major factor associated with both carbapenem and ␤-lactam resistance in endemic P. aeruginosa. Periods of extensive imipenem use were associated with significant increases in resistance. Our data support the concept that a written antibiotic policy which balances the use of various antibiotic classes may help to avoid disturbances of a hospital's microbial sensitivity patterns.Pseudomonas aeruginosa is a major pathogen in nosocomial infections, ranking second only to Staphylococcus aureus in a recent multicenter study of infections in European intensive care units (22). The organism may be involved in respiratory, urinary, wound, and bloodstream infections, primarily in patients with severe underlying diseases or impaired immune defense (3). Since the lethality of P. aeruginosa infections is high once the organism has caused disseminated disease, early and adequate antibiotic therapy is essential. Resistance of P. aeruginosa to antimicrobials used for primary treatment has been shown to correlate with an adverse clinical outcome (1, 9).While the incidence and risk factors for the development of antibiotic resistance in P. aeruginosa have been studied repeatedly in cohort studies or comparative studies evaluating the effects of novel antibiotics (2, 4, 11, 13), little is known about the influence of a hospital's antibiotic policy on resistance epidemiology in the hospital as a whole. In a 600-bed community hospital, we had the opportunity to observe sig...