Insuring that quality indicators really measure quality of care and not other factors, such as the type of intervention or the patients' characteristics, is notoriously difficult. In order to avoid as much as possible these potential methodological pitfalls, the association FoQual (www.hospvd.ch/quality/foqual) requested in the year 2000 the opinion of experts on the scientific value of some indicators, considered for introduction into practice by the commission on quality of care representing the Swiss hospital association and the health insurers' association (H+/CAMS), as well as on theoretical and practical aspects essential to guarantee their efficiency. The expert group Swiss-NOSO (www.hospvd.ch/swiss-noso) was asked to assess the indicator "nosocomial infection". This example illustrates some pitfalls to avoid, the importance of including infectious surveillance into a global prevention program and ask professionals with a specific training and independence from hospital wards to perform this activity. It shows the complexity of setting up and exploiting quality indicators in health care and the side effects that they might have.