The 41-year-old World Cup victor was diagnosed after falling ill on Friday and was rushed to the Can Misses Hospital on the Spanish island of Ibiza.
Sources subsequently confirmed the reports to ESPN FC, and later on Sunday the 41-year-old said in a post on his official Twitter account that he is expected to be released shortly.
The Brazilian legend asked for a transfer to private hospital called Clinica Nuestra Señora del Rosariro per respected island daily Diario de Ibiza.
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Staff at Can Misses Hospital said they were unable to give out any information for 'data protection reasons'.
He is a regular traveler to Ibiza and also owns a house in the island.
Ronaldo, who was on holiday in Ibiza when he was taken ill, retired from football in 2011 following a hugely successful career which featured stints at PSV Eindhoven, Barcelona, Inter Milan, AC Milan, Brazilian sides Corinthians and Cruzeiro as well as Real Madrid, where he is a club ambassador. Tissue swells, and it can cause coughing and breathing difficulties. Two of those were the only ones that Brazil scored against Germany in the 2002 World Cup final.
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Considered one of the best strikers of his generation, Ronaldo played for a host of top European clubs including Real Madrid, Inter Milan and Barcelona.
He finished his career with 12 goals in the World Cup finals, tying with Pele for the all-time record.
Week-end de chassé-croisé sur les routes
Ce week-end, le sixième de la période estivale, le trafic s'annonce une nouvelle fois chargé dans l'Hexagone. Samedi 11 août est classé ROUGE au niveau national et même NOIR en région Auvergne-Rhône-Alpes.