Gwent Police are appealing for information following a road traffic collision on the A465 between Tredegar and Merthyr Tydfil.
It happened at about 12.40am, on Sunday 29 December, and involved two cars, a black Audi and a silver Vauxhall.
The driver of the Vauxhall, a 29-year-old woman from Blaenau Gwent, was taken to hospital where she remains in a critical condition.
The driver of the Audi, a 34-year-old man from Swansea, was also taken to hospital for treatment.
He was arrested on suspicion of multiple offences including causing injury by dangerous driving and driving while unfit from drink and drugs. He has been released under investigation.
If you have dashcam footage, saw the collision or have any information that could help please call Police on 101, quoting 2400427214, or DM them.