This articles studies and describes the changes in the local party system of Medellín since 1988. To this effect, it analyzes three dimensions centered on the concept of institutionalization —system fragmentation, intra-party fragmentation and electoral volatility— by using quantitative indicators for these and other aspects of the local party and electoral systems. The study finds that there is a change from an institutionalized two-party system in 1988-1994 to a less institutionalized multiparty system after the 2011 elections. These findings contradict other studies on the national party systems and the standard institutional explanation by showing that transformations in the local sphere do not correspond to the changes in the electoral system nationwide. These results invite us to rethink the conclusions reached by this approach and highlight the importance of subnational comparisons.