A 19-year-old man suffered life-threatening injuries after jumping from a third floor window at the Queen's Medical Centre.
The incident happened in the early hours of Saturday morning, at an area near to the emergency department.
Visitors to the hospital on Saturday morning said they saw a smashed third floor window, and police at the scene.
A spokeswoman for the Nottingham University Hospitals NHS Trust said: "We can confirm that a patient remains in a critical condition following what is understood to be a determined attempt to jump from a third floor window at QMC in the early hours of this morning.
"Our thoughts are with the family at this time. Staff are receiving appropriate support.
"Access to our car parks via our Emergency Department was closed for several hours this morning while police carried out their enquiries.
"Main access to the hospital for patients, visitors and staff was via the Leengate entrance. The road has reopened."
Nottinghamshire Police confirmed the 19-year-old man is being treated for life-threatening injuries.
A spokesman for the force said: "Nottinghamshire Police was called just after 6.20am on Saturday after a 19-year old man fell from a window at Nottingham's Queen's Medical Centre.
"The man remains in a critical condition in hospital, where he is being treated for life threatening injuries.
"A police cordon remains in place within the Queen's Medical Centre building, as enquiries are ongoing to establish the circumstances surrounding this incident."
Sean Fay, 22, from Stapleford, had been visiting his grandfather in hospital when he saw the cordon.
He said: "There were two police cars here and it looked like they were kind of cleaning up.
"It happened just around the corner from the entrance to the accident and emergency department, near to the West Block.
"It looks like someone has jumped from a room which is three floors up. I went in to see my granddad, and when I came outside saw the scene and the window looked like it was smashed."
