Comet assay was performed on five racing horses (age, mean ± SD: 4.4 ± 0.5 years old) and five retired horses (10.2 ± 4.1 years old). DNA damage of whole blood samples were tested with or without UV irradiation. Three specimens were taken from each horse and one month apart for each specimen to test for reproducibility. Result showed that racing horses had statistically significant lower baseline and UV induced DNA damage in leucocytes than retired horses. Positive associations were also seen between age and baseline DNA damage or UV induced DNA damage from Spearman correlation analysis.