Police say they 'will not tolerate' trouble around Whiston Hospital following reports of staff being harassed.
Damage to property and cannabis smoking has also been reported to Merseyside Police.
The force said it is regularly called to the hospital car park and surrounding area over reports of "youths gathering" and urged parents to track their children's movements.
Such trouble was having a "massive impact" on staff and patients at the hospital at a time of huge pressure due to the Covid-19 pandemic, police added.
Police patrols are working with hospital security staff to combat the trouble.
Knowsley's community police team shared images of a force presence in the area on Tuesday night, writing on Facebook that officers "have been patrolling the area of Whiston Hospital this evening following a number of reports of youths acting anti-socially towards staff and visitors, using drugs and causing damage within the multi-storey car park".
The most recent incident was reported on Monday, though the suspected troublemakers had fled before the arrival of police.
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Knowsley Community Policing Inspector Peter Rexwinkel said: “We regularly patrol Whiston Hospital car park and the surrounding area due to reports of youths gathering.
"Damage has previously been caused, and we have also had reports of cannabis being used at the location, and staff and visited being harassed.
"The most recent incident was reported to us on Monday night, although no persons were traced when patrols attended.
“We won’t tolerate such behaviour and are continuing to work closely with the hospital and Knowsley Council's Crime and Communities service to respond to any such reports."
Whiston Hospital is part of St Helens and Knowsley Teaching Hospitals NHS Trust, which has so far recorded 375 hospital deaths linked to Covid-19.
Insp Rexwinkel added: "Such gatherings have a massive impact on those who work or visit the hospital.
"This would be a concern in normal times, but preventing this is even more important now, given the massive pressure our NHS is under, as well as the simple fact that people should be staying at home when possible. If you are a parent or carer for young people, please ensure you know where they are and encourage them to stay at home and not to mix outside their household."
*Anyone with information can contact Merseyside Police by calling 101, on Twitter (via @MerPolCC) or Crimestoppers, anonymously, on 0800 555 111
