A prospective cohort study was performed to determine the prolongation of stay and the extra costs incurred due to the occurrence of ventilator-associated pneumonia in intensive care unit patients. Over a 16-month period a sample of 270 consecutive adult patients from a large university anesthesiological intensive care unit requiring ventilation therapy for more than 24 hours was analyzed. A matching procedure using multiple control patients without pneumonia per infected patient (= case) was employed. Of 78 cases 21 (26.9%) died and were excluded from the matching procedure as well as 23 (29.5%) for whom suitable controls could not be found. The maximum number of controls per case was five. The mean added stay was calculated to be 10.13 days and the extra costs attributable to the prolongation of stay were 14,253 German Marks (US$8,800) per patient, demonstrating considerable added stay and costs due to ventilator-associated pneumonia acquired during intensive care. However, it should be taken into account that the calculations for excess stay and costs are based on a subset of rather ill patients and thus cannot generally apply to all ventilated patients and that cases were excluded which could not be matched.