Ronaldo began his international career with Portugal under-15 in 2001, playing in several friendly matches and in the European Youth Summer Olympic Festival The following year he would represent his country under-17 side at the 2002 UEFA European Under-17 Football Championship, where they failed to progress past the group stage. In June 2003, Ronaldo won the Toulon Tournament with Portugal under-20, where he scored one goal in five appearances. Ronaldo also featured in the Olympic squad at the 2004 Summer Olympics, scoring one goal in the tournament, though the team was eliminated in the first round, finishing bottom of their group with three points after 4–2 defeats to eventual semi-finalists Iraq and quarter-finalists Costa Rica. During his international youth career, Ronaldo would go on to represent the under-15, under-17, under-20, under-21, and under-23 national sides, amassing 34 youth caps and scoring 18 goals overall.
At age 18, Ronaldo made his first senior appearance for Portugal in a 1–0 victory over Kazakhstan on 20 August 2003. He was subsequently called up for UEFA Euro 2004, held in his home country, and scored his first international goal in a 2–1 group stage loss to eventual champions Greece. After converting his penalty in a shoot-out against England in the quarter-finals, he helped Portugal reach the final by scoring the opening goal in a 2–1 win over the Netherlands. He was featured in the team of the tournament, having provided two assists in addition to his two goals.
Ronaldo was Portugal’s second-highest scorer in the European qualification for the 2006 FIFA World Cup with seven goals. During the tournament, he scored his first World Cup goal against Iran with a penalty kick in Portugal’s second match of the group stage. In Portugal’s round of 16 match against the Netherlands, an infamously dirty game, Ronaldo was forced off injured in the first half, following a tackle from Dutch defender Khalid Boulahrouz. Ronaldo would accuse Boulahrouz of intentionally trying to get him injured after the match, although Portugal won 1–0 and he recovered in time for the next round. In Portugal’s quarter-final against England, Ronaldo’s Manchester United teammate Wayne Rooney was sent off for stamping on Portugal defender Ricardo Carvalho. Although the referee later clarified that the red card was only due to Rooney’s infraction, the English media speculated that Ronaldo had influenced his decision by aggressively complaining, after which he was seen in replays winking at Portugal’s bench following Rooney’s dismissal. Ronaldo went on to score the vital winning penalty during the shoot-out which sent Portugal into the semi-finals. Ronaldo was subsequently booed during their 1–0 semi-final defeat to France. FIFA’s Technical Study Group overlooked him for the tournament’s Best Young Player award and handed it to Germany’s Lukas Podolski, citing his behaviour as a factor in the decision. Following the 2006 World Cup, Ronaldo would go on to represent Portugal in four qualifying games for Euro 2008, scoring two goals in the process.