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, 41, was diagnosed with the illness after being rushed to Can Misses Hospital on the Spanish Island on Friday evening.
The two-time World Cup champion reportedly asked for a transfer to a private health facility identified as Clinica Nuestra Señora del Rosario later that night before being taken to intensive care.
Ronaldo is believed to have arrived in Ibiza for a summer break a few days ago.
Hospital sources have said he is still in intensive care but making good progress. The hospital where Ronaldo is recovering could not be reached by Reuters for comment.More news: Koepka takes 2-shot lead at PGA and goes for another major
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Ronaldo retired from football in 2011 after a glittering career with Barcelona, Inter Milan and Real Madrid.
Ronaldo, whose full name is Ronaldo Luiz Nazario de Lima, rose to fame for his exploits as a striker for many of Europe's top clubs, as well as the Brazilian national team. Tissue swells, and it can cause coughing and breathing difficulties.
In his latter years he battled with overweight because of a thyroid problem. Two of those were the only ones that Brazil scored against Germany in the 2002 World Cup final.
He picked up the Ballon d'Or award for the best player in the world in 1997 and 2002.