La Liga expanded from 18 to 20 teams in 1987, beginning with the 1987–88 season.
The change was decided during the 1986–87 campaign, when Spain’s top division was using an unusual two-phase format. The 18 clubs first played a regular phase, then were divided into championship, intermediate and relegation groups. Mid-season, the league and Segunda División were expanded to 20 teams.
As a result, the 1986–87 season had an exceptional structure and was one of the longest in Spanish football history. The following 1987–88 season was played with 20 clubs and returned to a more familiar league format. Each team faced every other team home and away, producing 380 matches.
The year can be confused with 1997, when La Liga was reduced from 22 teams back to 20. Thus, 1987 marks the expansion from 18 to 20, while 1997 marks a later reduction from 22 to 20.