The purpose of the present review was a comparison of the abortions caused by EAV and EHV-1 viruses over the 34 years. A total of 452 tissues samples from aborted fetuses (347) or foals (105) stillborn or newborn that died within 72 hours were investigated. The material for the examinations came from different farms located throughout Poland. The tissue homogenates were examined by using virus isolation test in RK-13 and Vero cell lines and the cytopathic agent was confirmed as EHV-1 by the direct fluorescent antibody test or as EAV by the indirect fluorescent antibody test. The study indicated that EAV was isolated (104 cases, 23%) almost as equally often as EHV-1 (116 cases, 25.6%). Both, equid herpes virus-associated abortion and the abortion induced by EAV were characterized by cyclicity. The percentage of EAV and EHV-1 isolation alternately reduced and increased, but the increase of isolation of one virus was accompanied by the decrease of the other. The domination of one virus over the other occurred in cycles of a few years.