SummaryAll patients in a Norwegian tertiary care university hospital were screened for hospitalacquired infection (HAI) on six dates during 2005 and 2006. The hospital costs for all patientswere estimated according to a national standard. A total of 159 patients with HAI and 2689control patients were identified. The median hospital cost for HAI patients was found to beNOK 381 716 (EUR 45 547) compared to NOK 153 512 (EUR 19 122) for controls.The excessive costs for patients with HAI were most pronounced in the second and third costquartiles. In the fourth quartile, calculated hospital costs were lower for patients with HAIcompared to controls. However, the uncertainty of the cost calculation increased with risinghospital cost. Median length of stay (LOS) was 20 days and three days respectively, butmedian LOS before appearance of HAI was six days, suggesting that the two groups ofpatients were not directly comparable. This was corroborated by the diagnosis related groups(DRG) weights which were 2.5 times higher among patients with HAI. When patients withLOS shorter than four days were excluded from the analysis, the cost difference between thetwo groups was eliminated.