Objective: To investigate an outbreak of nosocomial infection caused by multi-drug resistant Acinetobacter baumannii in a level 3 neonatal intensive care unit in Turkey. Material and Methods: Eighteen babies with multi-drug resistant A.baumannii infection and were treated with colistin between June 2009 and September 2009 in the NICU of Ondokuz Mayıs University. Infected case was defiined as patient hospitalized longer than 48 hours in the NICU, during the outbreak period with the clinical signs of pneumonia or sepsis and isolation of A. baumannii from tracheal aspirate, cerebrospinal fluid and/or blood culture. Cultures from the environment and staff were taken for the source analysis. Results: During the study period among 171 neonates followed in the NICU, 18 (10.5%) babies had pneumonia or septicemia due to multi-drug resistant A. baumannii and presenting mostly with respiratory symptoms such as tachypnea. During the same period, 21 babies died and 10 of them died secondary to A. baumannii infection. Among the patients with A. baumannii infection, 11 (61.1%) neonates were very low birth weight (birth weight <1500 g), and 14 (77.8%) neonates were born premature. A. baumannii in two patients were sensitive to carbapenems and 16 were only sensitive to colistin. Renal and liver function tests were followed during the treatment with colistin among 12 patients without any side effects. All infected/colonised babies were isolated. The staff’s cultures of hand were negative. Humidifiers of ventilators and oxygen manometers were determined by surveyans studies as the source of infection. New patient hospitalization stopped for 10 days. The outbreak ended at the end of September. Conclusion: This outbreak is the first A. baumanii outbreak in our unit. Previous publications reported the resources of A. baumanii outbreak as unit staff, air conditionars, and equipment of mechanic ventilators. We found humidifiers of ventilators and oxygen manometers as the source of the outbreak. We stopped hospitalization of very low birth weight infants until controlling the outbreak. As a conclusion advanced infection control measures, finding the source, cohorting patients, education of the team stopped the spreading of infection.