, doctors diagnosed a 'racing heart' when Ronaldo was in his mid-teens and it was feared the Portuguese winger, now the World Footballer of trhe Year, would have to give up playing.
He was playing for Portuguese side Sporting Lisbon when the problem emerged.
He underwent thorough medical examinations and it was decided to operate to solve his irregular heartbeat.
The condition is not uncommon and many high-profile footballers have suffered from heart difficulties.
Former Manchester City midfielder Marc-Vivien Foe died while playing for Cameroon in 2003, while France World Cup winner Lilian Thuram was forced to retire last year because of a heart problem.
Portsmouth striker Kanu is another with a history of heart concerns. However, fortunately for Ronaldo, delicate laser surgery cured his problem and enabled him to become the world's No 1 player.
Ronaldo's mother, Dolores Aveiro, said: "His heart raced a lot when he wasn't running.
"The people in charge at Sporting alerted me and I went to Portugal where I had to sign some papers and authorise him being treated in hospital.
"They used a sort of laser to cauterise the source of the problem. He was operated on in the morning and came out at the end of the afternoon.
"Before we knew exactly what he had I was worried because there was the possibility of him giving up playing football.
"But the treatment went well and after some days he was back at training again."