When the intestinal flora is susceptible, selective decontamination of the digestive tract (SDD) reduces colonization of the alimentary tract by aerobic Gram-negative bacilli, and ventilator-associated pneumonia is seen in units with a high rate of nosocomial infection. Although mortality is generally unaltered in studies of SDD, a number of deaths can be directly attributed to ventilator-associated pneumonia; no study has yet had sufficient power to demonstrate a reduction in mortality. Reasons for failure of SDD include infections by organisms that are acquired directly into the trachea thus bypassing the digestive tract, and failure to prevent colonization of the alimentary tract by resistant micro-organisms. As with any excessive antibiotic usage, prolonged use of SDD is likely to have an impact on the microbial ecology of an intensive care unit, thus making microbiological surveillance essential. The role of SDD at present remains controversial, but may be helpful in intensive care units where there is a high incidence of secondary infection.