He is a regular traveler to Ibiza and also owns a house in the island.
Retired Brazilian striker Ronaldo said Sunday he is in hospital with a "bad flu" on the Spanish holiday island of Ibiza but reassured fans that he will be out in a day.
Brazil World Cup victor Ronaldo is in intensive care in Ibiza suffering from pneumonia.
Later that night, he requested transfer to the Nuestra Señora del Rosario hospital owned by Policlinica, a private health-care company.
The newspaper's sources said the ex-footballer was still in intensive care, but was making good progress.
He retired from football in 2011 after a glittering career with Barcelona, Inter Milan and Real Madrid.
During his playing career, Ronaldo was famous for his blistering pace and phenomenal goal-scoring record. He is thought to have arrived on the island a few days ago before his illness started. The hospital could not immediately be reached by Reuters for comment.
In total, Ronaldo scored 15 goals at World Cups and won the competition's Golden Boot award for the top scorer on one occasion.
His full name is Ronaldo Luis Nazario de Lima.
Pneumonia affects the lungs and is caused by bacterial infection.
Symptoms - which include a cough and difficulty breathing - can develop suddenly over 24 to 48 hours.
In his latter years he battled with overweight because of a thyroid problem. It can be potentially life-threatening if severe and is particularly risky for the elderly or those with pre-existing conditions.