Cristina Fernandez de Kirchner has been ordered to take a month off work after bleeding was found on her brain.
The 60-year-old Argentine president was undergoing medical checks for another condition when the bleeding – a subdural hematoma – was discovered.
Vice-President Amado Boudou is flying back from France to take over her duties.
Cristina Fernandez de Kirchner has been campaigning ahead of congressional elections.
The break from campaigning is considered awkward for the president as some opinion polls have suggested the government could lose control of Congress in the poll on October 27.
Her spokesman, Alfredo Scoccimarro, issued a statement saying her doctors had carried out a brain scan in August after a previously undisclosed trauma – thought to be caused by a fall.
However, they found nothing untoward.
On Saturday, Cristina Fernandez de Kirchner went to hospital for checks for an irregular heartbeat and also complained of headaches.
A further scan revealed the subdural hematoma – bleeding between the brain and the skull.
The condition is said to be “chronic” but not “acute” according to the presidential statement, and Cristina Fernandez de Kirchner will not be staying in hospital.
Her doctors are to monitor the bleeding using imaging technology.
President Cristina Fernandez de Kirchner, a centre-left Peronist, was first elected in 2007 and then returned to power by a comfortable majority in 2011.
There have been hints that she may seek a third term, though Cristina Fernandez de Kirchner has never confirmed this.
This would also require a change to the constitution – only possible if her party keeps control of Congress.