A member of staff was stabbed as she arrived for work at Oxford's Warneford Hospital.
The 33-year-old woman was stabbed in the abdomen at 7.20am on Tuesday. She was taken to the John Radcliffe Hospital but later discharged.
A 61-year-old man was questioned by police and released on police bail pending further inquiries.
Police appealed for witnesses to the attack, particularly anyone who saw the attacker escaping over fields behind the hospital.
Det Insp Dermot Norridge of Oxford CID said: "This field backs on to Hill Top Road and the Southfield Golf Club. Anyone who saw anything suspicious should get in touch as soon as possible."
Contact Oxford CID through 01865 266333 or via the freephone Crimestoppers number on 0800 555 111.
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