Acinetobacter baumannii is a Gram-negative coccobacillus that has emerged as a troublesome pathogen causing institutional outbreaks. Environmental contamination is a distinctive characteristic of this microorganism, which brings a further difficulty in infection control. During A. baumannii outbreaks in intensive care units, a common contaminated object can be found as a reservoir. Finding out this source by epidemiological investigations is of particular importance in order to develop effective interventions. We describe an outbreak of A. baumannii and the results of epidemiological investigations in a neonatal intensive care unit. The outbreak strain was isolated from the outer surface of a breastmilk pump. We have successfully controlled the outbreak by careful reviewing of our milk collection process.