Staff at Can Misses Hospital said they were unable to give out any information for "data protection reasons".
Ronaldo is a frequent visitor to the Balearic party island of Ibiza and owns a property there. The hospital could not immediately be reached by Reuters for comment.
Symptoms - which include a cough and difficulty breathing - can develop suddenly over 24 to 48 hours. He is thought to have arrived on the island a few days ago before his illness started.
Ronaldo, who retired in 2011 had a glittering football career with top European clubs including Barcelona, Real Madrid and Inter Milan. Two of those were the only ones that Brazil scored against Germany in the 2002 World Cup final.
Ronaldo finished his career with an impressive 414 goals, 62 of which came in just 98 appearances for the Brazilian national side.
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