Two runners fainted and died on Sunday during a campaign to encourage children to exercise in Rayong province.
About 1,400 runners participated in the 10-kilometre race in Pluak Daeng district starting at 6am.
The two male runners — Narasit Sansompak, 54, and Sakan Janthiya, 30 — collapsed after running for four kilometres.
The paramedic team tried to save their lives but it was too late.
Rayong Hospital will perform an autopsy to find out the cause of death.