24 teams (the national domestic league champions from the best leagues, and a variable number of other clubs from the most important national domestic leagues). The competition culminated in a Final Four.
For the 1999–2000 EuroLeague, the countries are allocated places according to their place on the FIBA country rankings, which takes into account their performance in European competitions from 1996–97 to 1998–99.
FIBA: In Euroleague, teams finishing 6th in the Second Stage of 1998–99 FIBA EuroLeague (groups E-H) lose a berth for their country in next season. These berths are gained by those countries whose teams reach the Semi-Final Round in the 1998–99 FIBA Saporta Cup. As a general rule, a federation cannot have more than 3 teams in Euroleague. Should a semi-finalist in Saporta Cup provide one more berth for its country, in case it already has 3 berths assigned in Euroleague, this berth will be transferred to the next federation in FIBA Ranking which has 2 teams (as long as one of its teams reached the 1/4 Final Round in Saporta Cup during the same season)..