Objective:To determine the impact ofAcinetobacter baumannii(AB) acquisition in intensive care unit (ICU) patients on mortality and length of stay (LOS).Design:Pairwise matched 1:1 case-control study.Setting:Medical-surgical ICU in a tertiary health care institution.Patients:During 16 months, all patients admitted to the ICU were eligible. Case patients were defined as every patient with an AB isolation 48 hrs after ICU admission. Control patients were retrospectively selected from ICU patients without any AB isolation, according to seven matching variables.Measurements and Main Results:Attributable mortality and excess LOS in the ICU were measured. Eighty-seven patients were included, with 75 pairs successfully matched. Infection was defined in 48 patients (23 respiratory). The attributable mortality rate for AB acquisition was 30% (49% vs. 19%) (95% confidence interval [CI] = 23%, 37%): 43% (CI = 34%, 52%) in patients with infection (58% vs. 15%) and 53% (CI = 41%, 65%) in patients with respiratory infections (70% vs. 17%). The estimated risk rates for death were 2.6 (CI = 1.6, 4.5;p< .001), 4.0 (CI = 1.9, 8.3;p< .001), and 4.0 (CI = 1.6, 10.2;p< .01), respectively. The attributable excess LOS was 13 days for both AB acquisition and infection (23 vs. 10 days;p< .001) and respiratory infections (23 vs. 10 days;p< .01). In noninfected patients, no significant excess of mortality was found (33% vs. 26%), but LOS increased in 15 days.Conclusion:AB acquisition involved an excess LOS in ICU patients and increased risk of death, but the latter could be found only in patients with proven infection.