In order to monitor the use of antibiotics, it is essential to have comprehensive data on drug consumption. The findings of drug utilisation studies can serve to describe the pattern of drug use in a particular population, to detect areas of concer, and to evaluate the impact of interventions taken to influence the use of drugs. In the present study, the Anatomical Therapeutical Chemical Classification/Defined Daily Doses (ATC/DDD) system developed by the World Health Organisation was evaluated. The system measures the amount of drug used independent of package size and sales price, which allows comparisons not only within an institution but also within a region, a country, or even internationally. Obviously, there can be no modifications of thissystem. To illustrate the method, the pattern of quinolone use in the general population, in long-term care facilities, and within a single institution was analysed. These drugs were widely used in long-term care facilities in the Nijmegen region of theNetherlands, accounting for about 30% of the antibiotics used in these settings, whereas in the general population as well as in the University Hospital Nijmegen, these drugs constitute only about 6% of the total antibiotics used. These differences arelarge enough to warrant closer analysis of patterns of antibiotic usage in different settings to identify the reasons for the use of quinolones and to identify measures that might be taken to rationalise the prescription of these drugs.