From 1990, a study on the prevalence of nosocomial infections has been carried out yearly in Spanish hospitals. Acute care hospitals with more than 50 beds were involved on a voluntary basis. In 1990, 123 hospitals participated and by 1997 the number of hospitals had reached 214. The objective of the study is to examine the situation in each hospital, and to collect data across the country, by means of a common protocol. The overall prevalence of nosocomial infections has significantly decreased in Spain. The prevalence of infected patients has been about 7% in the three last studies. The prevalences for urinary tract infections and surgical wound infections have decreased, while prevalences for lower respiratory tract infections and bacteraemia have increased. Urinary tract infections have occupied the first position over the eight surveys. Second place was taken by surgical wound infections from 1990 to 1995, and by lower respiratory tract infections in 1996-1997. With the exception of Intensive Care Units, the prevalence of nosocomial infections has been decreasing in all hospital areas. The mean age of hospitalized patients has increased, so has the proportion of patients with one or more intrinsic risk factors and the proportion of those with one or more instrumentations. The proportions of patients with a short or a very long hospital stay have increased, revealing a change that no doubt reduces nosocomial infection rates. The use of antimicrobial drugs has shown a significant increase, from 33.8% of patients in 1990 to 35.8% in 1997.