The current investigation was designed to select infants in the neonatal intensive care unit (ICU) at highest risk of infection. The data, derived from a prospective study of 223 neonates, indicate that neonates in an ICU develop their initial aerobic pharyngeal flora according to one of the three patterns shown in Table 1, one of which (low titer or no growth) is dependent on antibiotic therapy. Of particular importance is the finding that infants with normal flora (alpha-streptococci predominating) or cultures showing low titers (less than 10(4) colony-forming units (cfu)/ml) or no growth did not become infected. All infections occurred in the group of infants with abnormal pharyngeal colonization (18 infections in 115 abnormally colonized infants): with a single exception the infecting organism showed the cultural characteristics of the colonizing organism. Consequently one can pinpoint the relatively small number of infants at particular risk of infection and predict what the infecting organism will be if infection occurs.