A single nucleotide polymorphism in the equine herpesvirus 1 (EHV-1) DNA polymerase gene (ORF30 A(2254) to G) has been associated with clinical signs of equine herpes myeloencephalopathy (EHM). The purpose of our study was to determine the odds ratio for this genetic marker and EHM using a panel of field isolates from North America collected over the past twenty-three years. EHV-1 isolates cultured at the Cornell University Animal Health Diagnostic Laboratory from 1984 to 2007 were retrieved along with their clinical histories. DNA was extracted from these EHV-1 cultures and allelic discrimination was performed using real-time PCR. The results were confirmed by sequencing of the target region in ORF30. PCR and sequencing were in 100% agreement and showed that 19 out of the 176 isolates had the ORF30 G(2254) allele (11%), of which 16 were EHM cases and 3 respiratory or abortion cases. The odds of having neurologic disease with the ORF30 G(2254) genotype were computed as 162 times greater than those with the opposite allele ORF30 A(2254) (95% confidence interval 35-742). Despite this strong statistical significance, 24% (5/21) of horses with neurologic disease in our study population harbored the "non-neurologic" form of the allele (ORF30 A(2254)), suggesting that other factors may also contribute to the onset of EHM.