Cyclists and motorbikers could have been badly hurt by pranksters tying metal wire across the entrance to a hospital car park, police have said.
The wire had been stretched across two pillars at the Royal Hampshire County Hospital in Winchester.
A police spokesman said they believed someone had taken the wire from materials belonging to contractors at the hospital.
He described the unknown vandal's actions as "incredibly irresponsible".
Inspector Kevin Baxman said: "Obviously, we can only speculate at the motives behind this persons actions but this type of stunt is incredibly irresponsible and could have serious consequences for motorcycle users.
"The wire cannot be easily seen and could cause a rider to come of their machine.
"It may be that the person who did this didn't realise the possible consequences of their actions and I hope that we don't see an incident like this occur in the city again."