Abstract. The role of antibiotics in preventing respiratory tract bacterial colonization and systemic infection after orotracheal intubation was prospectively studied in 54 newborn infants. Respiratory tract colonization was assessed from nasopharyngeal and tracheal aspirate cultures obtained at intubation and daily thereafter, while systemic infection was monitored by blood, cerebrospinal fluid, and suprapnbic urine cultures performed initially and every three days thereafter while intubated. Colonization and systemic cultures were also obtained at extubation or death. The study group, provided with antibiotics at intubation, and the control group were similar in birthweight and gestational age, as well as race, sex, hospital or origin, and indication for intubation. Colonization at intubation was five times more common in infants intubated 12 or more hours after birth than in infants intubated earlier. Subsequent colonization was twice as frequent in infants intubated longer than 72 hours as well as in those requiring two or more reintubations. Systemic infection occurred only in those infants who were initially or subsequently colonized and was three times more frequent in the control group than in the study group. [ABSTRACT FROM AUTHOR]