A mild outbreak of acute respiratory infection was reported in racing horses in the fall of 1995. Four studs were investigated for the sources and routes of infection. In five horses from two herds, virus isolates were obtained which, in preliminary typing experiments, were identified as the influenza A/equi 2 virus. The presence of this illness in all the examined herds was confirmed by a rise in specific antibody titres. The affected animals included both older vaccinated horses and young horses not yet vaccinated. Epidemiological studies suggested that the spread of infection occurred in situations where infected and non-infected horses were together, most frequently at races. Newly infected horses brought the infection back to their studs where further animals became infected. Some of them, still being in the incubation period without any clinical signs, may have taken part in another race and passed the infection onto healthy animals from other studs. Since the races usually took place in 7-day intervals, the epidemiological chain remained continuous. The causes of the outbreak of infection in vaccinated horses are analysed. The length of post-vaccination immunity, the antigenic composition of vaccines and their completion with new strains of the influenza virus are discussed.