US Secretary of State Hillary Clinton had a blood clot between her brain and skull.
Doctors said she will make a full recovery and will be released from hospital once a blood-thinning dosage has been set.
The BBC reports Mrs Clinton, 65, was admitted to hospital in New York on Sunday, having suffered a concussion earlier in the month when she fainted and fell over.
Doctors said an MRI scan on Sunday had revealed the formation of a "right transverse sinus venous thrombosis" - a clot in the vein behind Mrs Clinton's right ear in the space between the brain and the skull.
"It did not result in a stroke, or neurological damage," said Dr Lisa Bardack and Dr Gigi El-Bayoumi, who were treating Mrs Clinton at New York's Presbyterian hospital.
The secretary of state was said to be in good spirits and "engaging with her doctors, her family, and her staff," added the doctors in a statement.
A spokesman for Mrs Clinton said doctors discovered the clot during a routine follow-up MRI scan on Sunday. She is being treated with anti-coagulants.
Mrs Clinton was appointed secretary of state at the start of Mr Obama's first term, in January 2009.
She is due to stand down before President Obama begins his second term.
