The following season Karlsson played in Sweden's national under-18 team in the 1982 IIHF European U18 Championship, scoring two points in five games and winning a gold medal. Karlsson also played in Sweden's national under-20 team in the 1983 respectively 1984 World Junior Ice Hockey Championships. In the 1982–83 season, Karlsson was given the chance to play in Modo Hockey of the Elitserien, Sweden's top ice hockey league. During his first two seasons with the team he did not manage to score any goals, and in the 1983–84 season the team was relegated to Division 1. However, Karlsson and Modo were quickly promoted back to Elitserien the following season.