Sporting Clube de Portugal

Ronaldo joined Sporting's other youth players who trained at the Academia Sporting, the club's football academy, in Alcochete. He became the only player ever to play for Sporting's under-16, under-17, under-18, B-team, and the first team, all within one season. He scored two goals in his league debut on 7 October 2002, which Sporting CP won 3–0 against Moreirense, while featuring for Portugal in the 2002 European Under-17 Championship.^[22]

At the age of 15 Ronaldo was diagnosed with a racing heart, a condition that might have forced him to give up playing football. The Sporting staff were made aware of the condition and Ronaldo's mother gave her authorisation for him to go into hospital. While there, he had an operation in which a laser was used to cauterise the area of his heart that was causing the problem. The surgery took place in the morning and Ronaldo was discharged from hospital by the end of the afternoon; he resumed training only a few days later.

In November 2002, Ronaldo was invited to Arsenal's training ground, London Colney to meet manager Arsène Wenger and his coaching staff. Wenger, who was interested in signing the midfielder had arranged to meet Ronaldo's representatives, Formation (who suggested the player originally to Gérard Houllier, then Liverpool's manager) in the subsequent months to discuss a transfer arrangement.^[25] However he came to the attention of Manchester United manager Alex Ferguson in the summer of 2003, when Sporting defeated United 3–1 in the inauguration of theEstádio José Alvalade in Lisbon. Ronaldo's performance impressed the Manchester United players, who urged Ferguson to sign him. Ronaldo was offered to Barcelona for less than the €15 million Manchester United paid, but the Catalan club could not afford him at the time as the club had already spent its budget on other players.

On April 2013, Ronaldo was honoured by Sporting as the club announced that the player had become their 100,000th member.