According to reports in the western media, the former striker was admitted to Can Misses Hospital on Friday evening with chest pains where he was seen by doctors.
He tweeted: "Guys, I had a bad case of influenza here in Ibiza, I was hospitalised on Friday, but everything is under control!"
The 41-year-old former Ballon d'Or victor was rushed to hospital after feeling chest pains on Friday in Ibiza and according to local news outlet 'Diario de Ibiza' is now in intensive care. Tomorrow I'll be discharged and I'll go home! "Thanks for all your support and your messages".
Respected island daily Diario de Ibiza said he had asked for a transfer to a private hospital called Clinica Nuestra Senora del Rosario just before midnight the same day.
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The paper also said that Ronaldo was still in intensive care but is making good progress.
Ronaldo, whose club career took in spells at Cruzeiro, PSV Eindhoven, Barcelona, Inter Milan, Real Madrid, AC Milan and Corinthians, is on holiday in Ibiza, where he owns a house.
Ronaldo retired from football in 2011 following a hugely successful career which featured stints including Real Madrid, where he is a club ambassador. He represented his country in three World Cups, including the 2002 edition, when Brazil won its fifth tournament title.
He picked up the Ballon d'Or award for the best player in the world in 1997 and 2002.