In 2004, Meszároš competed in World Junior Championships in Finland. After losing their quarter-final match against the Czech Republic, Slovakia was then defeated 3–2 by Russia in the fifth place game. Meszároš scored a goal and an assist in six games. Later that year, he made his senior international debut with Slovakia in the 2004 World Championships. Meszároš recorded one assist in seven games as Slovakia was defeated 1–0 by the United States in the bronze medal game. At 18-years-of-age, he was the youngest player to compete for Slovakia in both 2004 tournaments.