After several successful years in the Italian and European championships, he made his 125cc world championship debut in 2004 as the team-mate of Roberto Locatelli. He finished his first season in the 15th place overall, and earned the rookie of the year award. His best results were a 3rd place in Qualifying and a 7th place in the Malaysian Grand Prix.

The following year Pasini was running in the lead in most of the races and won the Chinese and Catalan Grands Prix. He finished the season in fourth place overall. In 2006 Pasini finished fourth overall once again, having won the Italian Grand Prix and the German Grand Prix.

The 2007 season started very badly as Pasini suffered from motorcycles malfunctions early in the year. That lowered his chances of winning the championship. Despite having four wins, one more than the championship leader Gábor Talmácsi and as many as the runner-up Héctor Faubel, Pasini ended the year in fifth place.

In 2008, Pasini rode in the 250cc class for Polaris World Aprilia team. He gain his first 250cc victory in the opening round of the season during the Qatar Grand Prix and secured three more podium finishes in the first five rounds. However, he failed to maintain such a high form and finished no higher than fifth in the remaining races. He finished the season 8th overall and was the rookie of the year.

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Wikipedia