A prospective cohort study was undertaken with two end points: (i) to compare the 48 h time cut-off with the carrier state criterion for classifying infections, and (ii) to determine a time cut-off more in line with the carrier state concept. All patients admitted to the intensive care unit and expected to require mechanical ventilation for a period > or = 3 days were enrolled. Surveillance cultures of throat and rectum were obtained on admission and thereafter twice weekly to distinguish micro-organisms that were imported into the intensive care unit from those acquired during the stay in the unit. A total of 117 patients with median age of 61 years and median Simplified Acute Physiology Score II of 42, were included in the study. Of these patients, 48 (41%) developed a total of 74 infection episodes. Using the 48 h cut-off point, 80% of all infections were classified as ICU-acquired. According to the carrier state criterion, 44 infections (60%) were of primary endogenous development caused by micro-organisms imported into the intensive care unit. Seventeen secondary endogenous (23%) and 13 exogenous (17%) infections were caused by bacteria acquired in the unit. The carrier state classification allowed the transfer of 49% of infections from the ICU-acquired group into the import group. A time cut-off of nine days was found to identify ICU-acquired infections better than two days. These data suggest that monitoring of carriage of micro-organisms may be a more realistic approach to classify infections developing in the intensive care unit.