Lower respiratory tract disease developed in a group of racehorses in training between two and six years of age. Disease was observed in 22 of 25 horses for which full records were available. Seroconversion to Mycoplasma felis was demonstrated by indirect haemagglutination assay in 19 of 22 paired sera and high titres (≥64) were found in convalescent sera from the three remaining horses. Evidence of respiratory viral infection was confined to seroconversions to equine herpesvirus- 4 in two of the horses. Tracheal wash samples, taken from four horses with visibly increased tracheal mucopus, contained more than 104 colony forming units/ml M felis and high proportions of neutrophils. This is the first description of an outbreak of lower respiratory tract disease in horses in training associated with M felis infection.