In this randomized parallel study, we examined whether an acute ozone (O₃) exposure leads to increased DNA strand breaks in human lymphocytes. The groups were exposed to 0.21 ppm O₃ filtered air for two hours 30min and 4.5 h after exposure, DNA damage was determined in isolated lymphocytes using the Fast Micromethod. There was no detectable effect after O₃ exposure. We conclude that an acute O₃ exposure at the tested concentration does not lead to persistent DNA damage.