A probabilistic model to estimate the seismic risk of buildings is evaluated. For this purpose a specific methodology is proposed. The developed methodology allows explicitly consider important uncertainties that are present in the main elements, that are used to estimate the seismic risk of buildings. One of these elements is the seismic vulnerability of each building, which is mainly represented in the proposed methodology through probability density functions that describe the variation of a vulnerability index. In the developed methodology, the seismic vulnerability is considered as a property that is changing through the time. Therefore, it is possible to estimate seismic vulnerability curves for different stages on the future life of a building. The methodology was used to estimate the seismic risk of 59,905 buildings of Barcelona. According to the results, in average, 53,152 buildings have a probability lower than 5% of suffer some kind of collapse during the next 50 years. If the government of Barcelona conducts a program to do a seismic rehabilitation of buildings, then the first buildings that could be evaluated for rehabilitation purposes could correspond to the 1,317 buildings, which were identified in this work as the buildings with the highest seismic vulnerability.