Teenagers have been putting themselves at risk by breaking into a derelict cancer treatment hospital site which still houses radioactive material.
Cancer patients are now treated at St James's Hospital
West Yorkshire Police said they were concerned about people entering Cookridge Hospital in Leeds because radioactive materials were present.
The hospital was the main centre for treating cancer in the Leeds area.
Services have now been transferred to a new specialist unit at the city's St James's University Hospital.
Insp Simon Hepworth said: "There are signs around warning people of the dangers of entering the area, including radioactive material which is currently being removed.
"Despite this, we are still finding youths in the area putting themselves at risk.
"This is a dangerous place for them to be and I fear that if they don't listen to the warnings we could have a very serious situation on our hands.
"There are plenty of other places for young people to use in the area.
"Cookridge Hospital is not one of them."
Police said they would "deal firmly and positively" with anyone on the site who should not be there.