An anonymous threat led to Epsom Hospital being sealed off this morning the hospital trust has confirmed.
The hospital was locked down after the threat was made at 9am and people were let out about an hour later after the hospital found out it was a hoax.
A hospital trust spokesman refused to give any details of the threat but said: "To protect patients, staff and visitors, we temporarily closed the hospital.
"However, we are pleased to confirm it has now reopened following discussion with the police. The threat is not considered to be credible.
"We would like to apologise for any inconvenience caused to our patients and visitors."
A police spokeswoman said: "A safety concern was raised to Surrey Police by staff at Epsom General Hospital this morning. Officers have spoken with staff and offered them security advice around the issue."
But at about 10.30am people who had been let out were still not being allowed back in.
Louise Wheatley posted on Twitter to say she had been let out but had given up waiting to get back in.
She said: "Am afraid I know nothing! They didn't say anything."
Anne Whatley tweeted: "We're all out, they evacuated the whole building!"
A concerned relative, who contacted the Epsom Guardian shortly before 10am, said patients were not being allowed in or out of the hospital.
Charles McManus said his wife Gilly and her father, 90, were among patients not allowed to leave the hospital without being told why.
People affected took to Twitter, as rumours spread that the lockdown could be caused by anything from a bomb scare to a baby being stolen.
Mrs Cakery tweeted "Husband is stuck in Epsom hospital- apparently a security lock down where no one can get in or go out. Bit like Willy Wonkas factory!!"
Sam Earls tweeted: "What us is happening at epsom hospital we are on lockdown?"
Mr McManus said rumours had started to fly among patients and visitors, adding: "They thought maybe a baby has been stolen which is why no one is allowed to leave the hospital."
Epsom Hospital locked down