Yanukovych's falling sick with an acute respiratory infection is the latest twist in a crisis that has already seen him accept the resignation of the prime minister in a bid to placate protesters.
The sudden announcement in Kiev came after the pro-EU opposition vowed further protests in the ex-Soviet country's worst political crisis since its 1991 independence.
The outcome of the crisis remained unclear, with thousands of demonstrators still occupying Kiev's Independence Square, key streets and municipal buildings in the centre of Kiev.
The protests started in November as a push for EU integration after Yanukovych under Russian pressure ditched a deal for closer integration with the bloc, but have now become an all-out drive to unseat the president who until now has defiantly refused to budge.
"The president is on sick leave," Yanukovych's office said in a statement, without saying how long the leave would be for.
Citing deputy head of the State Directorate for Medical Affairs, Olexander Orda, the statement said Yanukovych had gone down with an "acute respiratory infection accompanied by an elevated temperature.