Slovakia became the 16th member of the euro currency zone, following Cyprus and Malta, which both joined in January this year. Slovakia will swap its koruna for the euro on January 1 2009.
Slovakia's bid was only one of the issues discussed by the leaders of the EU's member states.
European Union leaders refused to set a deadline for a deal on the stalled Lisbon Treaty after its rejection by the Irish in a referendum last week.
Irish Prime Minister Brian Cowen and European Commission President Jose Manuel Barroso stated that it was too still early to make a decision on the treaty's future.
The Lisbon Treaty was expected to come into effect on January 1, 2009. There are still doubts over the treaty's ratification by the Czech Republic. Under EU rules, the treaty, aimed at streamlining the work of the enlarged European Union, cannot enter into force if any of the 27 member states fail to ratify it.
Belgium, Cyprus, Netherlands, Italy, Spain, and Sweden have also yet to ratify it, although they are all expected to do so.
The summit in Brussels is also set to discuss soaring fuel and food prices, as well as the lifting of political and economic sanctions imposed on Cuba in 2003.