Fifteen unweaned thoroughbred foals, born on a stud farm to vaccinated mares, were clinically monitored during their first six months of life and repeatedly tested for equine herpesvirus type 1 (ehv-1) and equine herpesvirus type 4 (ehv-4). Nasopharyngeal swabs and blood samples were collected and screened respectively by pcr and seroneutralisation to detect the presence of the virus, explore its role as a possible cause of respiratory disease, and to assess the efficiency of the PCR for the diagnosis of this disease. The foals were divided into three groups on the basis of their clinical signs and whether they had seroconverted to ehv-1 and/or ehv-4: first, foals with no clinical signs of disease that had not seroconverted; secondly, foals with clinical signs that had seroconverted, and thirdly, foals with clinical signs that had not seroconverted. The results indicated that the viruses circulated on the stud farm despite stringent vaccination regimens against them, and confirmed their association with respiratory disease. The absence of significantly different pcr results among the three groups of foals showed that the pcr was effective in confirming the circulation of the viruses on the premises without being particularly helpful as a diagnostic tool.