Nasopharyngeal swabs and acute and convalescent serum samples were collected from a total of 115 client-owned horses in twenty-three outbreaks of respiratory disease reported in Ontario (October 2003 to October 2005). Sera were paired and tested for antibody to equine influenza virus subtype 1 (AE1H7N7), equine influenza virus subtype 2 (AE2-H3N8), equine herpesvirus 1 (EHV1), equine herpesvirus 4 (EHV4), equine rhinovirus 1 (ERV1) and equine rhinovirus 2 (ERV2). Overall the cause specific morbidity rate of equine influenza virus in the respiratory outbreaks was 56.5% [determined by the single radial haemolysis test (SRH)]. The AE2-H3N8 was isolated from 15 horses in 5 outbreaks. Interestingly, four fold increase in antibody levels or presence of a high titre against ERV1 was observed in 11/23 outbreaks and ERV2 was observed in 2/4 outbreaks where AE2-H3N8 was diagnosed as the primary cause of disease using the SRH. In conclusion, AE2-H3N8 was found to be an important cause of equine respiratory disease. ERV1 and ERV2, which were present in 18/23 outbreaks, may represent an important component in equine respiratory disease in performance horses.