Extra security has been drafted in at Alder Hey Children's Hospital in Liverpool to combat abusive smokers who are flouting new rules.
Staff are worried about the impact of smoke on critically ill children
The entire hospital grounds are no smoking since the national 1 July ban.
Staff say they are being subjected to aggressive abuse when they ask people to stub out their cigarettes.
They say smoking near entrances and by open windows is causing smoke to drift inside and that they are particularly worried about the cancer wards.
Reminder cards have been printed by the special no-smoking task force to hand to smokers explaining the rules but staff say they are having little effect.
Marcia Mercia, the hospital's acting chief nurse, says the staff feel threatened.
"Verbal abuse and aggression that has been displayed by members of the public is making it very difficult.
"They are causing lots of problems by refusing to abide by the no-smoking rules.
"They are smoking underneath wards where the smoke is drifting in.
"Our main concern is our oncology units where we nurse our cancer patients.
"People are smoking underneath the windows and it is causing us concern, especially for the children."
The hospital said that although they have employed extra security to patrol the site and remind families and other members of the public of the rules, they have also been subject to abuse and have had little effect.
Ms Mercia added: "We are not disallowing people to smoke, we just need them to move off-site.
"We need them to think of the children's health, their health and that of the staff here at the hospital."