One person has sustained life-threatening injuries and another life-changing injuries after a crash involving multiple vehicles, police said.
A further three people were also taken to hospital with injuries said to be neither life-threatening nor life-changing following the incident in Tonypandy, Rhondda Cynon Taf.
The crash happened on the A4119 bypass at Williamstown, just before 11:25 GMT on Monday, South Wales Police said.
A 19-year-old man has been arrested on suspicion of causing serious injury by dangerous driving.
A spokesperson for the force said he was also being treated for injuries believed to be neither life-threatening nor life-changing.
The road remained closed for several hours as part of the investigation.