The likelihood of finding evidence of inflammation in 551 tracheal washes collected endoscopically from 278 Thoroughbred racehorses increased with the number of bacterial colony forming units (cfu) per ml of wash (P < 0.001). The aerobic bacteria Streptococcus zooepidemicus, Pasteurella/Actinobacillus-like species and Streptococcus pneumoniae were significantly associated with lower airway inflammation whereas coagulase-negative Staphylococcus spp., alpha-haemolytic Streptococcus spp., Acinetobacter spp., Bacillus spp., Escherichia coli, Pseudomonas aeruginosa, non-haemolytic Streptococcus spp. and Enterobacteriaceae were not; Bordetella bronchiseptica was not isolated. Lower airway inflammation was particularly associated with bacteria in horses < or = 3 years of age. S. zooepidemicus, S. pneumoniae and Pasteurella/Actinobacillus-like species were isolated from 167 of 551 washes, either alone or in combination.