Twenty-seven newborns had an episode of late-onset sepsis (septicaemia or bacterial meningitis after 48 h of age) over an 18 month period. Preceding or simultaneous surface cultures were available from 26 babies. Colonization with the organism causing sepsis could only be documented in 10 cases. Colonization with aminoglycoside-resistant Gram negative organisms was common but there were only two cases of systemic sepsis with a resistant organism. Pseudomonas aeruginosa frequently colonized babies over the first 8 months of the survey, but subsequently virtually ceased to colonize babies, although it continued to be a common cause of late-onset sepsis. These findings do not support the utility of routine surveillance of organisms colonizing neonates in predicting bacteria causing late-onset sepsis. They also cast doubt on the value of eliminating colonizing organisms by expensive infection control measures.