Introduction: Loss of pregnancy in mares is a major cause of economic and emotional impact for horse breeders. It can have many different infectious and noninfectious causes. The aim of this study was identification of the main viral causes of abortion in mares in Poland based on tissue samples from 180 aborted foetuses submitted for testing between 1999 and 2022. Material and Methods: Tissues of aborted foetuses collected from different horse studs throughout Poland were tested for the presence of equine herpesvirus types 1 and 4 (EHV-1/-4) and if negative, for equine arteritis virus (EAV). The examination was performed using a PCR/reverse transcriptase PCR (1999–2012) and a quantitative PCR (2013–2022). Results: The cause of abortion was determined to be EHV-1 in 49.4% of cases (n = 89), whereas no EHV-4- or EAV-positive cases were found. The proportion of abortions due to EHV-1 differed between regions, with the highest percentage in the Lubelskie and Wielkopolskie provinces. Conclusion: The results of the study indicate that EHV-1 is the most important viral infectious agent causing abortions in mares in Poland.