Multidrug-resistant Acinetobacter boumannii is a challenge in the treatment and control of nosocomial infections. This retrospective study was aimed to investigate the prevalence of multidrug resistant A. boumannii in a respiratory intensive care unit (ICU), related risk factors and its impact on disease prognosis. Of 218 patients who were hospitalized in our ICU during the last two years; 37 (17%) patients (21 males, mean age 61.6 +/- 19.8 years) developed pneumonia and/or bacteremia due to multidrug-resistant A. baumannii. Previous antibiotic therapy was detected in 51.4% and hospitalization in 70.3% of the cases. Pneumonia (59.5%) was the most frequent cause of hospitalization and chronic obstructive pulmonary disease (21.6%) was the second one; 81.1% of patients had co-morbidity. Invasive mechanical ventilation was performed in 31 (83.7%) patients during the follow-up. Ventilator-associated pneumonia developed in 22 (59.5%) patients and bacteraemia in 9 (24.3%) patients. Multidrug-resistance was observed in 23 (62.2%) of patients. Highest rates of resistance (100%) was detected against piperacillin-tazobactam, ampicillin-sulbactam and ciprofloxacin, followed by imipenem and cefepime (78%), meropenem and ceftazidime (55%), cefoperazone-sulbactam (43%) and netilmicin (35.1). The rates of re-intubation and tracheotomy were higher in patients infected with A. boumannii compared to the control group (59.5% vs. 7.7%, p < 0.0001 and 21.6% vs. 3.9%, p = 0.001, respectively). There was no significant difference between two groups in terms of mortality, however, durations of ICU and hospital stays were longer in patients with multidrug-resistant A. baumannii infection than without infection (24.2 +/- 18.3 vs. 8.2 +/- 8.3 days, p < 0.001 and 33.3 +/- 19.8 vs. 15.4 +/- 11.4 days, p < 0.001, respectively). In conclusion, due to the high rates of drug-resistance in nosocomial A.baumannii isolates, the use of invasive procedures and durations of ICU and hospital stays exhibit an increasing trend.