In April 2003, the senior national team coach Morten Olsen called Kahlenberg to the squad for the first time. He made his debut for the national team, coming on as a 76th-minute substitute, in a 1–0 friendly win against Ukraine on 30 April 2003. In March 2004, Kahlenberg was called up to the senior squad once and made his second appearance for Denmark, coming on as a 78th-minute substitute, in a 2–0 loss against Spain on 31 March 2004. In May 2004, he was selected as the youngest Danish player for the 2004 European Championship. However, Kahlenberg did not play any games at the tournament, as he was placed on the substitute bench.