US Secretary of State Hillary Clinton has been admitted to hospital in New York with a blood clot, officials say.
Mrs Clinton suffered a concussion earlier this month after fainting and falling down.
At the time, she was reported to have had a stomach virus and to have passed out after becoming dehydrated.
Mrs Clinton, 65, is due to stand down as secretary of state before US President Barack Obama officially begins his second term in January.
Doctors discovered the clot during a follow-up examination on Sunday, her spokesman Philippe Reines said.
“She is being treated with anti-coagulants and is at New York-Presbyterian Hospital so that they can monitor the medication over the next 48 hours,” he said.
“They will determine if any further action is required.”
No information was given about where the blood clot had formed.
Mrs Clinton is due to give evidence before a Congressional committee in January in connection with the attack in September on the US consulate in the Libyan city of Benghazi.
The US ambassador to Libya and three American officials were killed in the incident.
Mrs Clinton was appointed secretary of state at the start of Mr Obama’s first term, in January 2009.